For those who are in contract negotiations, the signing bonus' are a violation of the "status quo". For those with a contract it is a violation of the contract unless the contract allows such compensation. People have tried to argue with me that the airlines are making the bonus offers before the pilot starts and therefore the contract does not apply. That is BS, there is nothing in any of the contracts that allow a company to pay certain pilots outside of the contract. Additionally, every contract has an implied covenant of good faith and fair dealing and using such a tactic to get around the contract would violate that.
I don't know why the unions don't want to push this issue. They need to simply file in court for an injunction to stop the companies from making these outside deals. If their lawyers are telling them that the companies can do this they need to get competent lawyers. I suspect the unions merely think its in their best interests.