The problem is they are 'buying worthless patents'. Then trying to get people to pay not because of the patent, but because the US court system is so expensive.

One of the problems is the patent office may grant an extremely narrow patent, but the courts just might interpret it broadly. However, the shell company suing you has no assets to recover over a bogus suit. So, taking them to court has zero upside, costs money, and has a tiny chance of costing a lot of money. Thus troll.