Well, yes, you are right. I don't think they are doing anything illegally, of course.
As I said to Mona, when the contract was signed, almost all of them were minors and I think hardly in the first contract idols or their parents make this kind of request. So If It isn't a company's politic to pay equally, then they aren't receiving equally.
The main priority for the members must be finally debuting and just in the contract renew, when they are already seniors and proved their value to the company, things might be different.
or maybe in certain recording contracts - at least I've read about some in the west (certainly don't know what kpop contracts are like) that are staggered...in terms of greater cut for the backend (ie. not the artists) until they reach a certain point
eg. first 1m albums sold artist (or producers) only gets 20% of profits (yes profits after expenses) but if >1m sold then artists (or producers) get a higher cut maybe???
which is what I've always said that contracts might be unfair (it's not surprising since the companies hold all the power) but if both parties consented (and in SKZ and most kpop idol's cases their parents) then it's a legal contract - BUT the performance of said contracts should be transparent - like if the contract is 50% company revenue, 10% artist, 40% other staff (producers, composers, etc) then the company should furnish monthly/weekly/quarterly accounts so that it is transparent what was earned or not earned!!!