Sega isn't tossing DMCA claims at people making fan games - they're encouraging fans to keep it up.
There have been a number of pretty impressive fan games of late. There was the Metroid 2 remake, the non-official Pokemon Uranium, and Green Hill Paradise Act 2. Of course, Nintendo issued takedown notices to the first two of those games, and last week, Game Grumps played Green Hill Paradise Act 2, bringing it into the public eye.
If you think that Sega was quick to toss out a DMCA claim of its own, think again. The official Sonic channel actually commented on the Game Grumps video, and said "Keep making great stuff, Sonic fans." This obviously made the day of the creator of the fan game, SuperSonic68.
Regardless of how you feel about fan games, you can't deny that Sega's approach is likely to have a more favorable PR outcome than just tossing out a DMCA claim. Fan games tend to not only drive interest in games in the franchise they're based on, and they also make the game's community more loyal and more excited. Kudos to Sega for getting it right.