They do have internet access, not some random radio update protocol.

When I wrote the software for the Coke and Samsung screens in Picadilly Circus in London (the times square equivalent) we used wired internet or brought hard drives in. We did actually have wifi access (firewalled though) to ground level.

All running Linux then. Alas i didnt get a photo of the one time we got a bootup screen on the sign with penguin after a power cut.