I can see some problems with having a pet crab wearing such a shell all the time, and it's certainly no matter of cruelty! A crab probably wouldn't be able to maintain a proper grip on the shell with its back legs, so it would quickly find that it would be falling out quite a bit. Then there's the problem of the fragility of glass; most well-designed tank layouts involve decorations that allow crabs to climb all over. Crabs fall all the time; if the glass shell were to break, the owner would be spending quite a lot of time trying to pick up the pieces. Not to mention, the crab would probably get its tender abdomen cut if this were to occur!

So in the end, wonderful art, it's cool to see how a crab fits into its shell. However, it's definitely not intended for practical use by the animal!