I once asked one of our Station Mangers how the airline could afford to give round-trip tickets for overbooked denied boardings. What he said was enlightening.

They are positive space, however, the number of seats availible is limited. So if you're holding one and want to travel on short notice, you're out of luck. To use these passes you have to plan a couple months in advance.

Additionally, they expire in a year.

Due to those two factors, only about 18-20% of those "free round trip" tickets ever get used. So, essentially its whitewash. It makes the traveler feel like they're getting something for nothing when they're really giving up something for a benefit that is essentially worthless.