I absolutely love Bettas, and we have three. Two are in 5g tanks, the other in a 10g tank. I believe they should not be in anything less than a 5g tank, although some 3 and 4g tanks may be ok.
They love to use all the space they can get, they're personable, and I just adore them....
I've seen some 2.5 gallon tanks and 3-5 mini bows that had enough footprint for bettas. As long as water changes are kept up I don't see a problem with anything 2.5 and up. Honestly anyone that rescues them from those cups and doesn't put them in a 1/2 gallon bowl is alright by me. Unfortunately it feeds the sale of them in that manner but the fact is once they're placed in that cup any method of getting it out is a service to that fish.