How many new parts would you expect to sell if you give used stuff away for free?
It's more than that Space. It involves 'economic value', 'intrinsic value', and 'waste'.
If the rental chains are 'paid for' with the rental, then HD could sell them as used, as-is, and that would be all profit. Might be more than their net profit on new chains. But it also keeps stuff out of the landfill.
Not everyone uses the 2 sizes of chain that HD uses on their rentals. In fact, I don't think that HD sells a saw comparable to the large Makitas they rent (except for the large Makitas they rent) - does anyone know if they even sell this chain in their retail aisle?. I know that they do sell a saw that uses the 3/8 low profile (56 or 57 DL) on their electric Makitas.
Only a small percentage of people know or would take the time to sharpen, re-size, and re-use them (people who otherwise might buy on-line anyways), so it might not cut into their retail sales.
As noted, there appear to be more economic incentives for making something and throwing it away, than for making something good, using it's full life, then recycling/repurposing it.
Philbert