But there is no logic or legal ground for trademarking the word "monster" or the green/white colours in a logo. It is all about bullying smaller companies using their much higher budget.
When you pay for an expensive legal team on retainer you probably need to get them to start bringing in money for the org to justify their size. From the article it seems like they just troll for any sort of putative copyright violation and just roll the dice.
Suing indie game developers is not a way to do it. There's a good chance these lawyers need to be paid much more for the time of dealing with this case than the devs ever had available.
Maybe most are small fish, but maybe some are big fish too. Maybe catching all of these small fish gives you precedence when you do go up to face some big fish in the future, so the benefits to these costs perhaps will just materialize in the future.