could be at times that there wont be 2 seats together in the back or the 2 that are available are Seat Guru 'RED' seats. So if the person gave up the UP they still wouldnt end up sitting next whomever it was they wanted to, unless they did as they did and offered it to the person who sitting next to their friend
I kinda feel that the UP belongs to whomever gets it and they are free to do with it as they wish.
I wouldnt have brought it up to a FA as I doubt they are well versed in what the Elite benefits are. Had I been that FA I too would have taken as they did, that You (OP) was simply trying to get the UP for yourself. If you really wanted Only to know what the correct procedure should be, you should have waited till you landed wherever and call OnePass and ask, in a general way w/o specifying any flight #.