Except UA would cost itself so much by selling those four tickets on flights that don't fill up that it would come out behind.

Furthermore, I doubt it would pass muster with the DOT. Even if it did, they're having enough trouble getting people to understand BE restrictions. You think that people are really going to understand this? I promise you, the first time they tried to enforce it -- "Sorry, sir, you purchased our 'VDB special.' You've been bumped; please come back Tuesday for your replacement flight" -- it would be an absolute disaster.

UA may not like raising the compensation, but they come out ahead. There's a certain amount of breakage on the certificates; and, even when there's not, they're still likely trading a discounted future seat for a high-fare current seat -- most voucher recipients aren't turning around and using them on full-fare Y.