What if the group is negotiating for something I don't agree with?
I suppose there are times when that's true. For instance, limiting the work-week to 40 hours, and after that time-and-a-half. You may prefer a 50 hour week or a 30 hour week. On the other hand, standardized employment packages have a huge benefit to the company (less negotiating) and to society (can make reasonable inferences about a person's work schedule). And, frankly, a lot of negotiations like that only work out better for you if most people don't do it, e.g. if everyone is willing to work 50-hours/week, we go from that earning you an extra 25%, to that being the new normal and the salary for everyone remaining the same.
But, long story short, I view that loss of freedom as similar to the loss of freedom living in a society. You miss out on some autonomy, but in return you get a range of benefits that should more than compensate you. And a voice (via elections) in what those benefits are. While occasionally you'll be screwed some, it's hard to imagine how a system could work that didn't do that to anyone, ever.
That particular right prevents anyone else from competing with them on negotiations, which in my opinion gives them an unfair amount of power.
It may be "unfair", but it is definitely the results of a free negotiation between two parties. Now, if you want to discuss limiting their freedom to enter into contracts with each other, that's fine. I don't think that freedom is absolute. But as most people professing your beliefs are fairly libertarian in outlook, I'd like to impose on you to explain why it's okay in this case. Especially since the right you think it is imposing on is your right to enter into a contract with one of those parties under your preferred conditions.
I'm certainly against any laws that would prevent them from doing that.
I believe the original intent was to try to damage unions by allowing free riders, and also make it harder to have a de facto union shop.