Another factor I believe might be working here is the 24-hour cancellation policy. I've noticed that my actual tickets charges tend to appear after the 24 hour window. If they do the $1 charge first, that validates that the card is good and then after the 24 hour window the actual charge gets applied. That way then don't have to get into the Chargeback processing loop. Everybody hates chargebacks, both merchants and processors and anything to reduce those is probably saves $$ at both ends.
Cheers,