What you describe as "compulsory" is a mandatory subject of bargaining.

If a company doesn't want the union to have this right then they simply should NOT agree to it in negotiations.

Nowhere is the law is union membership compulsive. If, and only if, A COMPANY and the union agree to a union security clause is union membership and dues required.

Why don't you rag on the companies who agree to this, rather than getting on the union's case?