The motive seems to be to give TC every possible advantage. Nothing inherently wrong with that, but I don't know if there's anything worth applauding.
Edit: just had a thought. If TC is going to (openly) start lying about respecting embargoes, what's to stop sources from lying about giving TC exclusivity?
Of course, companies and PR firms can choose to not bother with TC, but they're the 800 pound gorilla of the tech media. But would four 200 pound gorillas be better than one 800 pound gorilla?
It makes sense to threaten retaliation but actual retaliation does not. At least not without Robert Aumann weighing in.
If your PR firm's first, second, or even third, suggestion for you is to pitch to the biggest tech blog: fire them.
What a mess. My bad.
A startup could do it once before going on Arrington's black list, I suppose... although you'd have to get TC's competitors to agree to embargo the "exclusive" post until TC does theirs.
edit: what tlrobinson said