Not a huge Ticketmaster fan, but I actually don't have much of a problem with this as they have a complicated problem: allocate demand of a often scarce good (event tickets) in a way that's fair to non automated buyers. By placing a hold on the stock for a few minutes (rather than allocating at final checkout), it gives humans a fair chance to check out as their ability to secure the stock isn't based on how quickly they check out. Showing the time left to purchase is transparent to the buyer and eliminates confusion.