That’s the way my wife (an attorney) explained IP to me- basically, you have to defend it tooth and nail, because if you don’t, courts tend to find against you.
So in this case, the fact that people have been making Jayne hats for a decade or so with no legal challenges is probably powerful medicine against ThinkGeek or whoever sueing and recovering any money.
In all likelihood though, their tactic is to send the cease & desist letters, and then threaten to sue and/or sue with no intention of winning, but to just make it really onerous for the little guys so they’ll quit selling and settle.
(which is a big part of why I’m all for massive tort reform; the law shouldn’t be used as a weapon by the deep-pocketed like this)