PLEX is not a direct income for CCP. It is a kind of token for claiming service time (Omega) or cosmetics. The fact you can exchange it for ISK is irrelevant as CCP can only legally “book” it as an income when someone uses it in NES to redeem the token.
I’ve not made that up lol
So “buying” PLEX with cash is a kind of loan to CCP
And by exchanging it for ISK you cede your right to hold CCP accountable to whomever receives that PLEX from you.
And you don’t get ISK from CCP, you get it from other players lol