And what happens when the DimRats are no longer in power, and the NLRB is staffed with conservative appointees? That seems like it could backfire big time on the unions. 
Having been actively involved with NLRB rulings, they've always been handled on a political level rather than a contractual or reasonable interpretation.
Hell, one of the former heads, under the Bush administration would regularly go on talk-radio speaking about how much he hated unions.