The only other alternative is that Utah took a look at the penalties for not allocating tickets and determined they are worth paying. Seems less likely to me.

I understand the ticket request process takes into account membership level but with only 3000 tickets to the Holy War, even making them available on the public request site is not necessary to sell them out quickly. I can think of a few reasons BYU wouldn't want to post them there:

Unnecessary expense and effort
Mask the insane mark-up those tickets will inevitably have. I would guess $300-$400 per ticket. Save the headache of a public supply and demand debate.
Provide another unique benefit exclusive to legacy donors.
Package them in with other legacy exclusive benefits (tailgate, team hotel etc.)