It is not exploitative of the newer members being referred because the Referral Team mining bonus is an equal split between referrer and referee, i.e. both of them mine the same 25% bonus when both are concurrently mining.
There is no fiat money involved in the referral program. New members do not have to pay the referrer any money or pay the project any money to join. Pi is free to be mined by anyone.
The referral bonus only applies to 1 level of referral relationship. This means that the referral-grandchild’s behavior doesn’t affect at all the mining of referral-grandparents.