I don't think you should be allowed to take the tag away unless you have agreed a deal. Essentially they have screwed with his ability to get a bigger deal by stopping him taking a deal at the start of FA when teams have money to spend

I may be wrong and don't recall the implications of not doing so, but doesn't he have the opportunity to not sign the franchise tag?

Well if he didn't sign the tag, he wouldn't be able to play this season. No matter, the Panthers rescinded the tag; so that he may get signed somewhere else.