If that signing bonus is even true, how is that not a violation of the status quo any less than increasing hourly rates? I am no attorney, but it will be interesting to see how your union responds.
At least you guys know now that you have a nice, long lever to get contract negotiations finished with. The company is obviously VERY concerned about pilot recruitment if they're trying to come up with creative ways to get around actually negotiating a contract between trying to unilaterally increase the hourly wage, pay a signing bonus, and shut down your website. I hope you guys are able to take that lever and place it somewhere useful