Even I find myself thinking, and occasionally saying “PIN number”, but that’s because everyone else says it, now. What I was thinking was that those who started the rot possibly did it out of ignorance, not those who are sucked into it (to mix metaphors) by the natural imitativeness of the human species.
I can’t prove you are wrong, but I also don’t think it particularly important. When I look at “PIN number” or “ATM machine” or whatever, I see a broad pattern. This is how English works. It could be that the first person to say “PIN number” didn’t know what “PIN” stood for, but the fact remains that the usage fit the pattern. It sounds like natural English, so users of natural English took to it.
There is the idea out there in prescriptivist circles that if a usage arose out of ignorance this makes the new usage automatically bad. They aren’t serious about this, of course. Modern English, including of the most strict prescriptivist-approved sort, is built from layers upon layers of ignorant usages. As is usually the case, the cry of “ignorance” is a rationalization to justify a prejudice: not an actual strategy for analyzing language.