They will when they think it is worth using that tactic...or the threat of that tactic. The day 100%, or 50%, or even 25% of G day workers stop "helping out" at such a cheap rate will be the day the company will have to give something to get something back.
In the meantime, it is a permissible activity, as part of our COS, that pilots should be smart enough to realize is to the benefit of the company and detrimental the themselves, since our COS also has provisions for R days, which is where pilots needed to "help out" should come from. If there aren't enough pilots, then they can hire more so that everyone can get the G days they are entitled to. If there aren't enough pilots on "free" R days, then they will have to roster pilots for credit R days, as well as pay the hefty overtime rates rather than the silly G day callout pay.
Besides, most of us can read between the lines, as well as read the black and white print in our COS, to realize working on G days to "help out" is just screwing ourselves.