In America we plug things into other people's sockets all the time, and I think it's even an electrical code violation to make the socket unavailable.
While I'm sure they have the right to ask you to leave if they don't want you using their power, they can't have you arrested for it. It's even the same for a gun in most states, you can be asked to leave and arrested for not leaving after police show up, but not arrested for the gun itself. Granted, the gun example is not true in all jurisdictions, I Am Not A Lawyer etc.
Unless you're hurting someone or breaking something, the only punishment for violating one of these "rules" should be being asked to leave, and then being detained or removed if you refuse to do so or if go back after they told you not to come back on their property. Pretty simple stuff really, granted some areas have hard to deal with people; in a lot of the USA everyone is pretty friendly minus large metropolitan areas where they try to fix this by adding more rules.