With compulsory union membership, things are generally okay for the working class and bit less so for the business owners. With optional membership, those outs

With compulsory union membership, things are generally okay for the working class and bit less so for the business owners. With optional membership, those outside the union can be given better deals to encourage union members to quit. Once enough quit, the conditions can be eroded. Not improving non-union conditions is the best way to erode since the slaves don't realise they are being hurt. This is good for the business owners and marginal to bad for the workers. Without a union, you get a subservient slave class. Great for the business owners and rubbish for the workers.