The labor isn't the collective resource. It's the negotiating power.

The negotiating power only exists because of my participation. I am not destroying something that would exist anyway by my lack of participation.

Workers are weaker when scabs undermine their collective bargaining power.

Yes, when someone doesn't join your cause, you are generally going to be weaker. That's kinda how it works. Doesn't make it a tragedy of the commons though.

I think you're missing part of the story.

Which part?