That makes no sense to me. Certainly not for the Pats, because it totally ties their hands in FA. But it doesn't make sense as well. The Pats didn't hold a gun to Cassel's head and force him to sign the franchise tender. If he hadn't signed it, then any team would have been free to sign him for 2 first round picks, but he chose to sign the tender indicating that he is happy staying with the Pats even though there is no certainty that he will remain a starter. It will be very hard to prove "intent to trade" unless someone can show collusion between the Pats and Cassel. GB and Minnesota used the tag and then traded the tagged player last year, so there is plenty of precedent.
The fact is, any team that was willing to spend 2 first round picks to sign Cassel as a franchise designee can probably still get him for 2 first round picks now, they will just have to negotiate with the Pats. Every team has an equal opportunity to negotiate. Offering 2 1st round picks for Cassel before he signed the tender would not have guaranteed that he would have signed the offer, if it was from a team for whom he did not want to play. I just don't see how anything is substantially changed.