Why should they care to tell him to stop? He's not profiting more because of the name, and he's not causing them to profit any less. It's ridiculous.
They need to protect their trademark if they don't want to lose it.
Sure, if you look at any individual case it's just a little guy who's not costing them anything. but put them all together and suddenly you lost your trademark...
I thought that letter was awesome, but Starbucks did nothing wrong here, they're just doing what they need to do...