In sweden being an Union member is not free, dues are about ~220kr per month(20 euros) depending on your salary(this is what it used to be for unionen not sure about other ones, but probably similar).
It's fairly common to be a member of a union since it also has some nice advantages and swedes like unions.
You can imagine the strike-warchest get nicely filled this way :) .
In Germany you give 1% of your salary to the union if you belong to one.