What do labor unions have against private equity?

I'm not normally one for union-bashing, but I'm puzzled by organized labor's record of private equity-bashing. The New York Post reports that the two million member Service Employees International Union wants increased government oversight of the private equity industry, with a special emphasis on the various banks that are in desperate need of cash.

"The biggest buyout firms are used to gaming the system to turn a profit -- it's no surprise they want special rules now to take over another sector of our economy," SEIU president Andy Stern told the Post.

KKR and other buyout shops counter that the SEIU is trying to unionize employees at companies acquired by private equity, and is grasping at straws to drum up support.

That may be the case, but I can't imagine one has to do with the other. Employees should join unions (or not) because they feel (or don't feel) that their pay, job security and working conditions will benefit from membership. Bashing buyout firms would seem to be an irrelevant sideshow and a counterproductive one at that. Many union pension plans are large shareholders in banks and other firms that stand to benefit from private equity involvement, and they may be shooting their members in the foot by fighting macro issues like banking regulations that have absolutely nothing to do with their members' interests.