In a REAL LIFE auction, it is not so easy to back out. The auction deisgn of mule is not realistic in the first place. Therefore, by forcing someone to buy something, it completely changes the dynamics of the game.
Your not "FORCING" someone to buy it, they raised it themselves. If you don't want to buy something for that high of a price - don't escellate it that way. To me, it's not about just being "realisitic" it's about balanced gameplay. Right now the RICHEST person can stop anyone with less money from buying land - even tho they likely need it most, by using this tactic. The entire game is made to help the person in last, and hinder the person in first - while maintaining as much of a level playing field as possible. If someone has the money to run the score up - they should be forced to have a consequence of it.
In the land auctions, I am very much in favor that once the minimum bid is raised, at least 1 person should be forced to remain on the bid section. Thus if you are the one RAISING the bid, you risk everyone else backing out and forcing you to buy it. Heck, your the one raising it. And your not even forced to buy it at the HIGHEST level, but at the MINIMUM bid if you are on the screen alone. It is very much in the spirit of the game and fair play.