I don't think you thought this through real well. If Mondo were to sell 250 prints and people cancelled so that they only sold 240, they could just sell the rest of the prints since they are a physical object and not time limited. Not to mention they don't allow cancellations.
https://mondotees.com/collections/posters/products/inside-out-anger-poster.
Now imagine if Mondo sold 250 prints that could only be picked up at 10:55am on Saturday January 25th and if you didn't they would burn any unsold posters so Mondo eats that cost. One option would be for Mondo to not allow changes or cancellations (which they already do) which is what the airline COULD do so that they still get paid when you don't show.
However, United allows changes to flight plans so if you were to not show and they also don't get the full ticket price AND they didn't overbook then they would be left holding the bag for most of the cost of that seat.
Overbooking allows them to be a little more flexible. Mondo just won't refund you from what it says on their site. In that sense, Mondo is really scummy since they could just sell the extra product without any issues.