Is it common for doctoral dissertations from for-profit schools to be unavailable? I've been trying to find the dissertation of someone who advertises that they hold a "doctorate of management" from the "University of Phoenix". The degree requires a dissertation, but the school does not require the dissertation to be published, apparently, because neither is the work listed on ProQuest, nor does the University's own library hold it. My liaison indicated that the author may have deliberately chosen not to distribute the work. I have not yet been able to obtain a copy.

Is this obscurantist practice rare at good universities &amp; common at for-profit universities? 

Any other factors at play in the unavailability of a graduate dissertation in the United States?
I don't know about for-profit universities, but the practice at "good" US universities is generally as follows.  Before being eligible to graduate, the student must submit a copy of the dissertation to the library and to ProQuest.  By default, after being processed, it is then immediately available to the public from both sources.  (In some cases it might not necessarily be available online, or might require a fee, a visit to the university's library, or an inter-library loan request.)  

The student may be able to request an "embargo", in which case access to the dissertation's text is restricted for a period of time.  For example, this could be used if the student intended to commercially publish the dissertation or related material.  At my PhD institution, this period can be up to 2 years; any longer embargo requires the special approval of the graduate dean.  Even in such cases, I would expect that the library and ProQuest would still show the dissertation in their catalogs; they just wouldn't allow access to the text.

In your case, "embargo" sounds like the only possibly legitimate explanation. But if the student graduated several years ago, or if the university library and/or granting department cannot confirm the dissertation's existence, I would declare shenanigans.  (Maybe NSFW).