The problem is not paid signatures in general. When the merit system was implemented, juniors were removed of clickable signatures. But they can still have (non-clickable) signatures, and many bounties pay them for posting crap with those. So the spam didn't decrease (much?).
It is, according to the guy I quoted above. We don't want to punish the entire community by letting them participate in signature campaigns just because some bounty whores are getting paid to spam with Junior Member accounts. Maybe we can ban signatures for junior members or find another workaround.
Signatures up to junior member of even to member have to be removed completely, and there have been other many good proposals that they will never be implemented or will take ages until they do.
Most of those suggestions are honestly very biased and would only work in the favor of the high ranked accounts.