As such, it is very unusual for a Submitting author to have to pay an OA charge.
COI statement: I used to work for an OA-only publisher, but have not been employed there for the last 2 years.
I wouldn't say that it's rare to have to pay (even for a non-free journal)... in fact it may be more common to pay just based on the number of predatory journals and their published articles, it just isn't ethical (either for the publisher or often for the writer).
Open Access is a trend to combatting the walled garden model of scientific publishing, and is perfectly legitimate.
Scammy journals are taking advantage of authors' needs to publish a lot of work in order to advance in their field. They are "open access" not because open access is a scam, but because the journal is a scam and it's hard to get people to pay for subscriptions to a scam journal.
Yes. Even with "reputable" journals, the revenue flow often works like this:
1) The researcher does the research and writes the paper (usually at public expense). 2) The researcher submits the paper to a journal (often along with a hefty "publication fee"), where it is edited and peer-reviewed for "free" (i.e., also at public expense) by researchers at other institutions. 3) The researcher signs over the copyright to the journal. 4) The journal then charges other researchers, libraries, and the public (who have basically footed all the bills so far) another hefty fee if they want to read the paper.
That's the standard model. Beall's list was of journals that were abusive even by those standards.
Google ["page fees"], and you'll find a whole bunch of them.