Since you ask from a ethical standpoint, I would say this is fine. I assume when you have some reasonable small bandwith, this will hurt noone. When you want to run a fully fledged global marketplace of free video streaming, maby this is not the best way to do it. But when it is an information webpage, that serves a couple of thousand people a month, probably they will not recognize it in the supermarket.
Technical question: You have a supermarket close, where you have open networks, with no key nor connectinglimit?