the AP is perfectly willing to call the UKIP or Tea Party "terrorists"

Could you link to an article in which the AP calls the Tea Party "terrorists"?

while refusing to call groups like Hamas terrorist.

The term terrorist is often used to describe nonstate actors. Hamas is govern and that makes it a state actor.

That's basically the same justification of why the CIA employee who isn't a military solider isn't a terrorist when he bombs rescuers against the laws of war with a drone.