There's no policy issue here. OPUP means "operational necessity of the air carrier." DL needed one seat in steerage so they offered the seat to you. If they had needed two steerage seats, possibly your companion would have got an UG.

But, these aren't complimentary UG's in the sense that domestic F cabins are simply filled with freebies until full. DL doesn't give free UG's as a policy matter, using them only when it needs them.

Your options were as you stated to offer the UG to the guy who got your seat, possibly your companion or to decline it and let someone else have it.