If you make a chocolate bar that looks too much like a Cadbury's one, you'll be in trouble.
I think this shows where the problem lies. The Cadbury's trademark applies to a fairly narrow set of products, whereas the HBO one covers "clothing, mugs, drinking glasses, hats, bags, mouse pads and similar tat" *. The HBO trademark applies to a commonly-used sentence on a wide group of products that often carry messages.
If I sell a chocolate bar in purple foil, then it's reasonable to claim that it's counterfeit Cadbury's. If I sell a hat with "Winter is coming" on it, but no knights in armour and women with their tits out **, then it's absurd to suggest I'm passing it off as an HBO hat.
* I suppose it's too much to hope that the registration actually uses the phrase "similar tat".
** I'm afraid my knowledge of Game of Thrones is entirely second-hand.