I felt oddly vindicated when he mentioned Ticketfly specifically... It irked me too that they don't allow guest checkout, so in making http://showboarder.com I put in the extra thought/effort to make it an option.
That said, having implemented this I can see why many services don't. It's an entire other case to consider when thinking about security, user experience, etc. and for many it could definitely be more trouble than it's worth. It's just a question of values if you make it a priority or not.
I'd suggest that the guest use case is the most important one and that account generation/creation/etc is for most sites the use case that should come second.
That said, having implemented this I can see why many services don't. It's an entire other case to consider when thinking about security, user experience, etc. and for many it could definitely be more trouble than it's worth. It's just a question of values if you make it a priority or not.