Do you know for a fact whether those that don’t join FSS do so because of the cost, or because they can’t be arsed, or they assume someone else will do it? Or that everyone who has joined is rolling in cash?
Even if that is demonstrably the case, it still remains factually true that FSS members are bearing a bigger burden. They do so voluntarily, and maybe because they can afford it or maybe not. But the fundamental point remains true.