Not necessarily. Tampering accusations in the past have been brought in order to force teams to pass on a player. i.e. by suggesting there has been "tampering" and putting the Jets under the microscope you force them to be significantly more cautious and maybe even pass altogether on Crabtree. If they think Crabtree's agent is in fact counting on this, it also reduces Crabtree's leverage with the Niners.
Of course, that's only a relevant tactic if you actually think the Jets are actually tampering (since otherwise what's the point of making a team pass on a player they don't even want to begin with?) But they don't have to be able to prove it for the accusation to be effective.
IIRC, this is what happened with the Green Bay, Vikings, Jets saga a year ago, which resulted in Favre's 1 year exile to the Jets before finally hooking up with his preferred choice, Minnesota. I know this sort of "tampering accusation to block another teams pursuit, regardless of proof" has occurred in the NFL before.