Why do journals reformat your submissions? When submitting to (computer science) conferences, you usually have full control over the layout and are able to submit a camera-ready version that will be published as submitted.

For journals, however, you usually have to submit a manuscript that will be reformatted before publication. In my experience, this includes rearranging and scaling of the figures, reformatting the equations etc.

Of course, it made sense before researchers were able to typeset camera-ready papers, but now it seems to be unnecessary and just generates additional costs and errors. Why do publishers still prefer this way instead of letting the authors do the layout? I would expect that the journal style can easily be enforced by providing an appropriate LaTeX template and some additional rules.

I thought it might be due to copyright issues (maybe if the publisher does not do any modifications, it has no right to have a copyright on it), but it also happens for open-access journals where this should be no issue.
I can think of a few different reasons:


Not all authors are able or willing to conform to the formatting style. After all, academics are supposed to be experts in their field of research, which usually isn't typesetting. The journal provides formatting as a service to these (and other) authors, and to ensure a consistent style.
Copy-editing can also fix a number of typos - it doesn't always just introduce errors...
Even if the authors manage to nail the formatting and spelling, chances are that the journal has an extensive style guide that goes way beyond "some additional rules". If you submit papers to more than a single journal, will you really be able to remember which journals insist on capitalizing words like "ansatz" in English?
Additionally, journals may use headers/footers they don't want to make available in public templates, or even unusual page sizes that may require the text to be reflowed.
Publishers may accept manuscripts in several formats, and have to convert at least one of them into their own format of choice. 
I don't know how common it is, but the format of choice need not be one of the accepted formats at all. E.g. APS accepts LaTeX and MS Word submissions, but converts both into an XML format. This allows them to generate both PDF and web versions of the papers, and additional versions in the future. There is even work underway on using the XML format to make papers fully available to the blind.


Finally, as long as the author signs over some or all of their copyright, such issues should not be a reason for this. Also note that formatting/editing does not necessarily cost that much, especially compared to carefully checking a submission.