Originally Posted by
Ditto
But I could have just as easily picked 24E.
Of course they could then refund the person in 24D if they paid for an empty middle seat but that seems a bit stupid.
They wouldn't offer you the option of choosing 24E if the person in 24D had paid for a middle seat. It's not clear from your screen grab that you were actually able to select 24E in that specific case.
Regardless, it would be a very easy system tweak for them, in the case where 24D bought an empty seat 24E but 24F is still available, to also offer the person considering 24F the option of buying an extra seat. KLM already knows it will remain empty (because 24D paid for that), but that doesn't preclude them from inviting 24F to also pay to have 24E empty.