The way you say THE system implies that lining up for a ticket is the PROPER system, or the way "everyone else" uses.
The situation is the same as if he physically bought the tickets two weeks ago, but missed the show due to traffic. From memory, "everyone" pre-bought tickets many years ago.
I agree that Assassin buying his tickets online opened up the possibility for his paying for something he didn't see, but that doesn't make the realisation of it his fault.
In any case, there are simply two systems in place. Neither is THE system that one should take. Your implied comparison to queue jumping is flawed in that it's not an endorsed way of purchasing something.
Assassin also adhered to your "first come, first serve" idea, since there are two points of sale: counter and website.