You don't need people to have Premium to change their skins. You just trick all the other players into thinking they are looking at a different player by sending them a different name(Client gets the skin themselves)Agree with Raneo,the skin is coded to the premium account called _infected_,so everybody would have to buy or have premium minecraft (this was suggested for tnt).
This is it exactly. Each name has it's own skin. You can't get one without getting the other.I've seen people ask about this before, and I'm nearly certain it's impossible. The name goes with the skin. You can't have the _Infected_ skin without the _Infected_ name, if that makes any sense.