Most products you pay for when you get them. If they run out they run out. Maybe you get it later, maybe you simply don't get to buy it. Occasionally you get your money back if they can't provide.
However, most travel products involve things where you prepay for something with limited capacity where substituting a later date is not anything like the same value. Thus failures are far more likely and procedures have been put into law to reduce the harm to those who can't get what they ordered. I'm surprised nothing has been done about rental cars--the lack of one can be a total trip-wrecker.