The riders are racing, they shouldn't have to be looking out for motorbikes. The motorbike riders have a responsibility to pass through the race safely and keep out of the riders way.
That dude didn't, and, arguably, interfered with the outcome of the stage. That's not on.
Fair enough, the backward facing footage didn't really show anything conclusive (as I said at the time).
Still, two riders not really looking where they are going and ending up swerving towards a motorbike that was swerving towards them.
Footage here: http://bleacherreport.com/articles/2529773-jakob-fuglsang-finishes-stage-18-in-5th-place-after-being-hit-by-motorcycle