Why is league of legends so harsh with their bans?

Because people are trying to enjoy their game, and there's no reason to ruin that to have the same childish environment fostered by games like Call of Duty; Riot is being better than that, and having some restraint and respect for your fellow players should be the norm.

They aren't even harsh. Everyone gets plenty of chances before the punishments escalate. It's not so much "Riot" deciding what to punish as it is the players, though. The community drives the system through their reports.

Pretty vague question.. depends on the situation. Most of the people who are banned usually deserve it because either they are being toxic, or trolling/griefing.

also there's 68 million players, they can afford to lose a few