This is essentially a private property rights issue, and the owner/operator of a privately owned and maintained repeater is entirely within their rights to grant or deny permission to use their property, the same as with any radio transceiver equipment. The licensed owner of any radio station is still responsible for monitoring and governing the transmission traffic according to the FCC regulations, as is anyone who transmits on the airwaves within US jurisdiction. Penalties for abuse can range from revocation of any license issued by the FCC to civil and/or criminal penalties depending on the nature of the offending behavior. Besides that, it is only a common courtesy to ask for permission to use someone else's private property.