Imagine the situation where every single company in your state is a union shop (you have to join the union and pay dues if you work) - and every single union is wildly pro-gun, supporting candidates who want everyone to carry two guns (but they'll compromise to only men and only one if pushed).
So to live and work, your money has to go to support that. If you were anti-gun, you'd probably feel a bit annoyed.
Guns are a right but only to a limited extent. No right is absolute. The debate right now is how far this right extends.
Side note - where I am (Australia) compulsory union membership is illegal.
At its core a trade union is an organisation that represents workers interests, and that formally negotiates on behalf of its members. To say that unions should be illegal is to say that workers should be banned from collectively negotiating via formal organisations, which seems a tad extreme.
However, I'm sure there would be other ways (for example, if the union is negotiating with the company, the company can just pay for the union, it does indirectly through salary anyway).
Voluntary membership is a hard problem for unions, and in Australia declining union membership is a big thing outside of industries like construction that still have unionism as a core part of the working culture. I think it's hard for a lot of people to see the benefits they gain from union membership when most of the basic working rights that unions have won are taken for granted, and a lot of the bargaining and political lobbying work they do is somewhat abstract.
It always seems strange to me how much anti-union sentiment there seems to be in the US though. Not to say that Australia is perfect by any means but labour conditions in the US seem pretty insane as an outsider.
That way if the union does a good job, workers will join, and if it does a bad job they won’t.
So to live and work, your labor has to go to support that. If you were anti-capitalist, you'd probably feel a bit annoyed.