Yes, it's people who break social contract. The sin of AirBnB isn't that they create those people where they didn't exist before. The sin of AirBnB is that they organize those people on a massive scale and put them in a position where breaking the social contract is the default behavior.
Tourists are noisy and careless. That's to some extent forgivable - tourism is a leisure activity. Hotels and other legit tourism-oriented businesses are prepared for it and mostly take care of it (and are appropriately regulated to make sure they do). But take the same tourists doing the same things and sprinkle them around neighborhoods where regular people live and raise families - in that context, their behavior becomes disturbing and destructive to the quality of those neighborhoods. It's breaking social contract.
The sin of AirBnB is that they know they're making the world a worse place, and they don't care. They just keep doing it.