This has been widely discussed here -- it's definitely not an FAR, although possibly an ops spec for some airlines (including UA), which is a serious pain to change.

I don't like the way the plastic cups taste. If they had a keg of budweiser and I was at the company picnic, uh maybe, but especially sparkling wines in cheap plastic -- I will sometimes take water in the plastic cups and never really like it.

I was really annoyed on a recent UA flight in C, of the two champagnes on the menu, they were pouring the good one pre-departure in plastic, and I declined, then once in flight had the worse one. I just don't get that.

To clarify, since there was some confusion: yes CO and UA (and AA) both have glass once airborne, all the time in my experience. I have not yet seen plastic in F in flight, although I've heard some of the lesser airlines have it.