If you want to work at a company there are lots of aspects of corporate culture and working environments that you have to accept if you want to work there. Presumably if you don't want to work in a union shop... you won't?

And to put another way: would it be reasonable for you to benefit from the work the union does for all employees without also contributing to that union?

All this is besides the point that requiring employees to join the union is a decision the _employer_ would have to make (although probably through union pressure).