How does collective bargaining force people to join unions?

heritage foundation says that if we require collective bargaining, firefighters will be forced to join the IAFF, which does not allow members to serve as volunteer firefighters. as a result, it says, volunteer fire departments will lose the core of their firefighters and will have to shut down, causing financial and safety issues... i just don't see the link. if its to get their voices heard (the only reason i can think of), well, with a union as big as the IAFF, wouldnt their voices be ignored anyway?