The leadership of Coast FA has been beating this drum for many years, even trying to rally for the cause. It's kind of strange since they are members of both, and dual roster their players, so this really shouldn't be an issue for them. It became an issue for them when they tried to get others on board for a league sanctioned under US Club for younger teams, to include an alternate state cup. This league had fees associated with it. It didn't take off as they had hoped because there were other teams/clubs that didn't want to dual roster players to be able to have the insurance coverage, since they would be participating in unaffiliated play.
This has always been about the insurance. The $50 per non USYS sanctioned team for participation in a USYS sanctioned tournament ensured insurance coverage for that team. That $50 per team was paid to SCYSA. Different clubs have approached this different ways. Some adjusted the entry fee and paid the SCYSA out of total receipts from the tournament, rather than pass that additional fee directly to the "unaffiliated" team that wanted to participate. Other clubs, as noted in the exhibits have chosen to directly charge that team directly.
In the end, follow the money, and you'll see why adults are suing. I agree with BigDaddy, each one of them will say it's about increasing the development of the players.