You say it is federally protected and not rude. I, as the interviewer, respect your interest in working for a fair wage but think it's not part of the interview process. We can discuss and you can turn down the job once it's offered because of pay but please don't ask me that in the interview. I don't care about the feds and at the point you ask me, you might just move to down the list of attractive candidates. How are you going to prove that I moved you down the list because of that one question? You can't and thus the federal protection is stupid and useless.

Take your chances though and maybe you will find a like-minded individual that agrees with you and doesn't find it rude. Won't be in my office though. If I can't find pay info before I apply somewhere I won't apply. Pretty simple. But I won't ask until a job is offered and an offer letter is given with the full pay and benefits package spelled out.