Some do. Mainly, the theaters where the bar is not in the theater itself (the Golden, for example). You just have to ask.
If you reuse your Jujamcyn cup at any Jujamcyn theater, you'll save $5 on your drink. I'm pretty sure the bartenders are told to explain that, so they can't refuse to refill it when you bring it back.
Also, DON'T put those cups or their lids in the dishwasher. They will warp.
It seems like a cheap money grab (and the cheap plastic cups probably are overpriced), but they are meant to reduce messy spills in the theater, and it does help a lot.
Nothing matters but knowing nothing matters. ~ Wicked
Everything in life is only for now. ~ Avenue Q
There is no future, there is no past. I live this moment as my last. ~ Rent
"He's a tramp, but I love him."