I don't think it's laziness so much as that there is no real benefit to them taking a side. One of the reasons that the Sims is so popular is because it allows user created content. People like it because to a certain extent they can shape the game as they see fit. While they can bring their lawyers down on paysites (and trust me, I'm sure EA's legal team is just as lawsuit heavy as Blizzard's), it serves no useful purpose to take a side and alienate customers.
You still have to buy the game to use paysites' products, and according to EA, technically anything made for in game use belongs to them and they can use and take it as they see fit (thinking of the stuff packs, I'm sure we've all seen some very familiar content in those, yes?).
But what do Blizzard gain from suing people who sell stuff with an orc's face on it? It's just free publicity. Sure, people are making money off of their artwork, but aren't paysite owners doing the same thing?