This issue has been raised many times over the years. You'll also see it raised with respect to hotel loyalty programs (particularly Hilton and Marriott). As others have noted, UA makes no promise of upgrades at the window, they just state upgrades "may" be processed at the windows. The language is thus similar to marketing fluff used by businesses in innumerable other contexts. Also, UA does occasionally still process at the windows, thus pretty much destroying any chance a plaintiff might otherwise have of winning a claim for deceptive business practices.
Good luck with that (and I mean that in both senses of the phrase, i.e., (a) I'd love to see someone win this argument, but (b) there's basically no chance of prevailing).