[fjsolwmv]

Same reason that people who live in the US don't have the freedom to choose whether they want to be governed by the US government. Social contract

[humanrebar]

But on a more concrete level, union fees go to campaigning, lobbying, and donations to candidates the union members may not support.

I don't think it's part of an American social contract that you would be forced to tangibly support candidates who have (by some accounts) immoral positions on abortion or immigration (to pick to polarizing issues).