Why do support items still have diminishing gold when fully completed?

So based on my experience by the time support items are fully completed (1,000 gold quest) laning phase is either over or just about over and we begin transitioning to the midgame where occasions arise where the support may be required to clear or help push a lanes. If they begin killing too many minions however they begin getting signifigantly reduced gold per kill. I understand why this is necessary during the laning phase, to prevent ADCs and other lanes from also abusing the support itemization in the same way jungle items are gated but why does this carry on AFTER the item is already transformed and it no longer grants it's gold passive?
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