Right-to-work proposes to ban private businesses from proposing or entering into such a contract. But there is no law requiring employees to join a union.
Having a unionized workplace is a bit like having a democratic government – in the U.S., we're born and become citizens of the U.S. government and all that comes with that. We can't "opt out" unless we move out of the country and renounce our citizenship. We still have to pay taxes and we get all the benefits of our government (safety, education, roads, etc.) even if we don't want to – you can't just not pay taxes, right?
If you don't want to join a union workplace, just find a non-union workplace. If you don't want to join a new union, vote against it and find another job if they accept the union? That's more choice than you get with being a citizen of a country where you pay taxes! (And usually, union dues are less than the negotiated benefit/wage increases anyway.)
They are saying that non-union workers are bound by the union's decision and have to pay into it.
You are saying that this is a good thing.
Almost always as a rule they are. You are not going to get people to vote for a union security clause that decreases net pay.