I'm not comparing them to humans exactly. An opinion is an opinion. Snakes can get fat, and thus need exercise. Its the same with all animals if you feed them and feed them and they never move, eventually they become a fat, boring lump. I got into garters because they are more active, they move and from my experience they need to move. I don't really care what your experience is with them. My garters move around a lot. It seems cruel to put them in a tiny container and say that they're happy.
My snakes are well fed, they move a lot after eating and they have muscles, what happens when something doesn't use it's muscles? They disappear. They have to work their muscles if they're to stay strong.

Again, this is my opinion. If you don't like it, then too bad. My snakes are 100% healthy in their large, spacious and decorated tanks.