I mean what other choices do they have?
UMASS going to the AAC is pretty much not going to happen, #1 is UMASS athletics budget. I don't know what it is off the top of my head, but I can't imagine it being much better than most MAC schools. #2 is because once Navy joins in 2015, they're at a very nice set of 12 members. Unless AAC is interested in extending an invite to another team in the future, I don't see this happening at all.
Mountain West? Currently have 11 members, so adding a 12th isn't out of the question, and MWC basketball is pretty solid from top to bottom, so for UMASS hoops it may not be such a downgrade. Whether or not the MWC is actually interested in taking UMASS ifs debatable.
They might have a chance at CUSA. I THINK (there are a lot of moving parts this year) that East Carolina, Tulsa, and Tulane are all going to AAC, which would bring their total to 13, but then they add WKU this year to make it 14 teams.
Since WKU is leaving the Sun Belt for CUSA, they'll be down to 9 teams. So if they want to make it a nice even 10, they could add UMASS.
I would have to agree with OUcats82 though. I hope that the Sun Belt makes an "all or nothing" offer, because I don't see them benefiting from this move very much unless they can get UMASS' hoops programs, but the Sun Belt is pretty much on par with the MAC overall as far as competitiveness goes, so I don't see that happening.