It's really very simple. The strong teams finish syn goal very fast; some of their members then go out to assist weaker teams who might have more trouble getting there. Gree wants those weaker teams to buy gold, not get help from players who have already bought gold for themselves or have a big pile of bonus gold sitting on their account.
I'll bet the only players who have called for syndicates to be locked for SA are the tiny minority of mean-spirited people who don't like the strong helping the weaker. None that I've seen on these forums though.