I am wondering how a practice of an employer requiring non-union members to pay union dues would be legal. Doesn't this constitute a tax?

Edit: it looks like this practice is not legal in right-to-work states in the US. This article seems to explain "Union security agreements" pretty well if anyone is interested:

http://www.nolo.com/legal-encyclopedia/free-books/employee-r...

I wonder are non-union workers in union shops ever pressured to join?