> Unions can't force their members (or non-members) to strike, except for the fact that if as a member you refuse to join a strike organised by your union you might be expelled.

Unless I am heavily misreading, “we organized a strike, and if you don’t join us, you are expelled from the union” sounds exactly like a union forcing its members to strike.

What other kind of “forced to strike” scenario were you imagining? Union leaders holding those refusing to strike at a gunpoint? Because other than extreme examples like this one, it sounds like a textbook version of forcing members to strike.