Predictably, $3 billion is simply a negotiated discount on fees in return for Foxconn choosing Wisconsin over a state not willing to discount fees.
1. There's a reason that foxconn wants to manufacture in the USA, and there's a reason they're courting states in the great lakes region. Resolving the Prisoner's dilemma becomes much easier when there are ~3 states with viable sites as opposed to 50.
2. See guelo's comment.
Your point is well-taken in general, but in this particular case, IMO, it really is a stupid giveaway.