Historically it wasn't uncommon to attempt to physically prevent strikebreakers from crossing the picket line. Most if not all organisations have standards of

Historically it wasn't uncommon to attempt to physically prevent strikebreakers from crossing the picket line. Most if not all organisations have standards of conduct that they expect their members to follow, and its not surprising that unions view strikebreaking ("scabbing") as being contrary to those standards. But seeing as the whole point of unions is organised action, why would you want to be a member if you weren't interested in participating in that action?