My initial gut reaction would be that it's their fault for leaving their bikes laying down on a mountain bike trail where, by it's very nature, people would be riding mountain bikes. Especially so on a red/black run where people will naturally be riding faster.
Unless they had a very good reason for their bikes to be there (ie. if they'd both fallen off them), then there's no excuse. If you're not riding your bike on a trail, move it to one side so that others can. You wouldn't just park your car in the middle of the road, so why should it be any different if you're out riding..? It's selfish and it ruins it for others.
You could argue that ajantom was riding too quickly, but then again he was on a red run which is designed for that very purpose so I don't think he's doing anything wrong. It's the two ar*ehats who decided to create the temporary roadblock which is the problem here.
Personally I think the guy is lucky to get away with a taco'd rim and not a taco'd nose.
Being a red run is not a screaming fast run, the slightly more experienced but still learning will be on them. Black is a different matter. But that was not part of the initial topic.
Many 'trail centre' red runs end on paths, fire roads etc. You can never expect them to be clear.
It could easily be somebody just getting off their bike, walking across etc.
Unfortunately other people use them, if no rules of conduct are given by the owners then how are they to know, they're learning. It's just an accident.
The car example is not the same, cars owners have rules they are suppose to follow even as to how and where to park. Many choose to not follow them, the code of conducts are there for all to read.