If this is harmless and poses no risk to the casino, then why do casinos always find ways or if they find out players are circumventing the rules, continue to ban them?

I think it depends, as if it is a registered casino and is strictly regulated by law enforcement. They need to be strict about this, they can't just go after small profits and may pay a heavy price if law enforcement finds out they are being lax in managing their users. Casinos also have risks and that is why they are very strict about this although it cannot be completely prevented as players will have many ways to cheat, IMO.