What is the reasoning behind the not rusty warning?

While I agree with the sentiment that it is technically stealing. No one should worry about charging their phone in public. Atleast in the region of the US I am located, it costs about 1-2 cents per year to charge your phone. So charging your phone for one sitting would be a miniscule amount of money. Just opening the door of the business and letting the conditioned air out would cost them more.

Obviously cars are an entirely different situation since one charge can be several if not tens of dollars.