Why do that when you can engender so much ill-will from all the people playing Scrabulous on Facebook?
There's actually a couple of very good reasons.
The first is that in buying Scrabulous, that opens the door for everyone to take Hasbro's trademark, infringe upon it, and then make money when Hasbro buys them out. Thus, there's a never-ending string of implicitly allowed trademark infringement for money. Why would Hasbro, or any company for that matter, do that when they can spend one small sum of money and lay down the law? It would be a small sum too - like as not, any lawyer would be able to win this case, since it's a clear example of trademark infringement.
The second is alluded to by the first. Suppose someone infringes on a trademark, and makes a crappy version of one of Hasbro's games, with a very similar name. It may be for the purpose of getting bought out or not. By buying Scrabulous, Hasbro has set precedent for this to be okay, and any lawyer worth their salt will argue that Hasbro can't cherry pick good applications and claim infringement on bad ones. In essence, it has to be an either/or issue. Additionally, while we'd all like to have hope for humanity and think people would be able to tell what is endorsed by Hasbro and what isn't, that never seems to end up being the case. Heck, that's a big reason why trademark was invented in the first place.
If someone coded a game called "Dungeons & Demons" that copied all of the rules of D&D and could be played online, we wouldn't be having this discussion. Scrabulous is no different - people just like to say/think it is because they enjoy it. That's too bad, Scrabulous being popular doesn't change the fact that they infringed on the copyright, and probably made money off of it.
