Nemo's a man with no home but all of the world's oceans. Yeah, we don't feel too bad for him. When we read 20,000 Leagues Under the Sea, we don't know exactly why Nemo left dry land behind. Sure, it had something to do with the death of his family and tyranny and evil men. But we do know that he has no intention of going back, so his exile is more self-imposed than anything else. In fact, he's gotten so good at living in the ocean, he's only got sea legs these days; putting this dude back on solid ground would give him a true taste of banishment from his homeland—er, homesea.
Nemo's exile is less a punishment than an opportunity to start fresh.
By casting himself out of society, Nemo has everything to gain and nothing to lose. From the moment we meet him, we realize that he no longer had any good reason to remain in the "civilized world."