How can publishers justify author fees for open access journals? The context for my question is that I recently discovered a new journal called Experimental Results, published by Cambridge University Press. Their mission, as stated on their website, is: 


  Experimental Results is an Open Access journal providing a forum for experimental findings that disclose the small incremental steps vitally important to experimental research; experiments and findings which have so far remained hidden. Such results often go unpublished due to the traditional scholarly communication process, in which only a select group of experiments are chosen to make up the narrative of a single paper.
  
  Articles for consideration in Experimental Results include validation
  and reproducibility of existing findings, null results, supplementary
  findings, improvements or amendments to published results, as well as
  results that could be of importance, but for whatever reason, the
  researcher has not followed a particular line of questioning to
  produce a full narrative for a traditional paper. Where applicable,
  work published in Experimental Results will clearly link back to the
  related article.


While I applaud the idea of open access and publishing the science that Experimental Results is targeting, I can’t say that I understand how Cambridge University Press can justify the absurd 700 EUR article processing charge (ACP). How can they rely on authors’ good intentions to pay that amount to publish &lt;750 words (the stated word limit) of science that is largely procedural? 

Furthermore, when publishers houses see >35% profit margins on their “products” by relying on faculty service obligations for manuscript reviewing (among other factors), why does Cambridge University Press (one of the oldest and most well-established scientific publishers) need to charge a fee for this, at all? Why can’t it be truly free and open access?

Speaking generally, I do understand the common justification for fees relating to open-access publishing (i.e., someone's gotta pay, and if it won't be the university libraries, it's going to be the authors) - but this seems like it has gone too far, in my opinion.

To summarize my specific questions for clarity:
1. What is a reasonable justification for author fees in journals like this?
2. Why would authors be motivated to pay such fees to publish, instead of freely sharing information?

(Also, I am new to asking questions on SE, so I apologize if this type of opinionated/targeted question is inappropriate or violates policy by lacking a "correct" answer. This just seemed like a well-informed community in which to have this discussion.)
The short answer: some journals use author fees to offset the costs of running open access. 

Over the past decade, two practices have become more common together in academic publishing: full open access journals and author fees. The STM Report, an overview of STEM publishing, says the practice is growing in their 2018 edition, "3.2.1 Gold APC and author fees": 


  A growing number of journals make their entire contents freely available immediately on
  publication (full open access journals) using supply-side payments. Generally, these
  payments take the form of an article publication charge (APC) levied by journals at the point
  of publication, and it is up to authors (or frequently their research funder or institution) to
  make the payment.


In other words, traditional academic publishing has consumers (research libraries, universities, independent researchers, databases) paying for access to a journal. Open access means that the consumer side is effectively free. Instead, publishers recoup costs through  "supply-side payments" like author fees (known as a Gold APC [article publication charge] model) or institutional sponsorships (known as a Gold no APC model). Authors' fees have been around for decades in various forms, including "page charges," but this report and other commentators (like Kent Anderson) observe that these fees are on the rise again. 

What is involved in that cost? The report also includes an answer, under "2.9.12 Overall costs of peer review," emphasis mine:


  The notional global cost of peer review is substantial, albeit largely an estimate of academic time  devoted  to  it  rather  than  actual  cash:  an  RIN  report  estimated  this  at  £1.9  billion annually,  equivalent  to  about  £1200  per  paper  (RIN  2008).  The  Houghton  report  used  a slightly  higher  figure,  at  £1400  per  paper  (Houghton  et  al.,  2009).  These  figures  are  full costings, including estimates for the time spent by the academics conducting the review. The publisher’s  average  cost  of  managing  peer  review  (salaries  and  fees  only,  excluding overheads,  infrastructure,  systems  etc.)  was  reported  by  the  PEER  study  at  $250  per submitted manuscript (Wallace 2012).


So Cambridge University Press may be using the author fees to offset staff salaries, fees, overheads, infrastructure, and systems. Some of their cost is related to peer review, and some would be entailed in running the journal itself. Even if the actual peer reviews from experts are unpaid, all of the surrounding work incurs a cost. Given the RIN report and the PEER study estimates, that cost is somewhere in the hundreds or low thousands of dollars, pounds, or euros. 

How does that affect submission rates? As you might guess, some authors are turned away by fees. That said, these publications still tend to get submissions, perhaps because author organizations are able to pay the fees, because the pressures to publish outweigh the costs to the author, or because they are attracted to the open model for ideological reasons. In other words, the motivations vary from author to author.