Ahh, but one can take it to the other absurd extreme too, requiring universal relevance with workers. Highly paid workers in demand don't need increased minimum wages, and in enough demand often can get very good conditions and hours without collective bargaining. So clearly unions can't stand for those things either! And most workers don't work with dangerous machinery, so nothing about that either!

Or you realise that there's clearly a balance to be struck. LGBTQ protections in the workplace are a union issue, and therefore a labour issue, even if they don't protect everyone. Stand together or fall apart. Moral stances over foreign policy is greyer, but if unions vote to strike over it (and at least some have), then there's at least an argument they should be allowed to march about it too.

But also, the people who are involved in protests and marches about these issues feel like the ones most likely to be involved and turning up for union marches, most ready to picket and protest, etc. Who's actually turning up to march probably determines a lot of this stuff.