If you're publicly accusing them of doing so, and doing so for monetary gain, then yeah, it does seem like some measure of journalistic integrity would require at least a half-assed attempt at getting the other side of the story. Even TechCrunch shoots out an email so they can say that they asked the company what their side is.

If you're really searching for the truth, you should have approached them and asked, especially since your title is a question and you have this line in your post:

I can’t figure out why 37 Signals would be selling this information to ad networks.

Your post is making an implication that something nefarious is going on, without actually having the guts to come out and say it. Instead, you pretend to be confused, and state that you're really not sure what's going on. If that's actually the case, why not just ask them?

As it is, it appears that you think you do know what's going on, you want to stir the pot and get some publicity for your product, and by keeping it vague, you protect yourself if you're wrong.