Right to Work

Unions have done some great things to help the working class rise up from the oppression of the "owners".  People fought and died to have the right to join a union.  Now, people have a right to join a union, without interference from their employer, yet in many states, they are not allowed to say, "This union isn't doing me any good, I don't want to be a part of it".  I don't get it.  What's wrong with being able to opt out of union membership?  If the union has something worthwhile to sell, they should have no problem with membership.  If they don't have anything worthwhile, why should people be compelled to buy?