Welll...think about it...
If a person is bed ridden for months and months...and say one day they magically feel better....and try to stand up and walk. They wouldn't be able to. Their muscles would have weakened.
Look at veal and other pharm raised animals. They are hung in hammocks often. There have been rescues where they release the animal from the hammock - only to have it fall over.
What makes a fish different? It would also have to deal with a different water pressure than it has ever known , as well as a filter current, as well as being with other fish (if its in a community tank) when it has probably never even SEEN a fish other than a betta, let alone being in a tank with one.
Then, like I said, when they were fed in their cup, the food just sits right on top. Still. In the new tank, they have to swim, say, a foot up, and the food is buzzing all around the surface of the tank - plus it would have to compete with other fish to eat.
Putting a betta into a tank is like introducing it into a whole other world. The only thing thats the same is the water they're in (and even then its likely to be a shock to have water that is *gasp* clean!).
Thats just my two cents...