Initial justification for TPAs were because there was big money from rugby and english super league leading to a bleeding of players (sailor, Rogers, thorn, tuqiri etc). But now Roosters, Storm, Broncos all seem to be able to attract big talent on small contracts.
I think power brokers at those clubs have an influence on NRL policy and they want this dodgy system maintained. There's no way this TPA system fits with the intention of the salary cap and this is obvious in that the top teams maintain dominance in the period since the TPAs came into play.
Your assessment of TPAs is very naive and it is not the way big clubs have used it. Given super league and rugby are nowhere near the threat they were there is no justification for this system except the powerful clubs have too much influence over nrl policy.
This is a messy system that should be changed if integrity is of any importance to the nrl.