Probably OSHA guidelines not keeping up with technology. Technically they're operating machinery on-site. There's probably a line somewhere that states that if you're operating machinery on-site you need to be wearing the appropriate gear.

The concept of driving a vehicle while not being in the area wasn't a possibility they considered while writing the regulations.

And yes, you could say that they should "just use common sense", but that's a fuzzy line. There's often good sounding excuses as to why one should not follow the regulations to the letter "this time". And those are the times bad stuff happens.