Because to survive as a business SO needs to sell your sense of 'comunity' to a third-party resource.
Not that there's anything wrong with that, just be aware that when you're participating on SO you're only a 'human resource', to be marketed and sold sometime later in the future. Hopefully SO is run by decent guys and the buyer will be ethical.
By "sell" you're implying that the sense of community is in some way diminished by selling advertising. I don't find this to be the case, unless it leads to a conflict of interest (i.e. SO deletes all Linux articles at the behest of Microsoft).
Now if you were to use "capitalize on", "monetize", or one of the many verbs that don't imply a transfer of ownership, then I would agree with you.
Not that there's anything wrong with that, just be aware that when you're participating on SO you're only a 'human resource', to be marketed and sold sometime later in the future. Hopefully SO is run by decent guys and the buyer will be ethical.