Last I was in one it wasn't nearly that much, but that was a couple decades back. My best guess is that so many places try to kill the unions by passing 'right to work' laws where people are not obligated to join even in a union voted shop. It makes it pretty tough to force a good deal when only a fraction of the workers are in and paying while they probably live in an area where people aren't concerned about working across a line.
This way all members can participate in a strike and give it the necessary strength to achieve something. Still most Union fees are ridiciously low. Usually they are in the area of 1,5% of your salary.
