I was at Sage when RMA module came into inception. I seem to vaguely remember this comment made in the design meeting, paraphrased: ""For the new RMA, in order to allow a return you have to allow backorders first and to that end we will not allow 'Allows Returns' until 'Allow Backorders' is selected."" And so from that point on, Allow Returns was always disabled / grayed out to begin with.
IOW, ideally the logic to disable / gray out Allow Returns should check if you have RMA installed and activated first. If no RMA then no need to initially gray out Allow Returns.