Isn't the union for loading and unloading different from union for shipping crews? Why'd these two unions cooperate?
You are correct that the stevedore unions are different from the seafarers unions.
The issue is that they cooperate legally or illegally. They also cooperate internationally.
Its the brotherhood above all else.
(and I use the gendered language intentionally because I have never come across a more misogynistic group of people in my life)