"the idiotic "use it or lose" clause in it. "
That requirement is in virtually every country's trademark laws. The idea being that you can't submarine in a trademark claim after something becomes popular.
I've seen cases where the company and lawyers who were involved were actually apologetic, but explained that if they don't enforce in every case, it has the potential to remove protection for the ones they are enforcing - and with HBO asserting trademark in all the paraphenalia associated with a show that pretty much means anything and everything.
But yes, they're tossers. They could have sent a much nicer letter in the first instance.