I don't know if this is necessarily the case but rental cars are a popular way to move drugs - they don't get identified with a known dealer or appear on a database as they're not with an owner for long enough. As such it might be worth the expense of a tint to keep the insides private
Of course I might be way off base and people just like hiding from the heat, but if that were the case then I'd expect more cars to be available with the tinting to start with.