It doesn't really matter. It's easily available distraction and people will end up looking on movies instead of the road all the time. It's like with TV in your car. It's not obstructing your view on the road, it's too easy to focus attention on it thus it's illegal to have one (unless it's switched off over some speed or w/e details various countries/states implement).

>I imagine a responsible Glass user could do the same.

The problem is many users won't be responsible. Good driver who doesn't use phone when driving is focused and well trained could responsibly break speed limit very often and still drive safer than 99% of people out there. Unfortunately it's impossible to fit the law to such cases so people who pay attention are hold to the same standards as texting-while-driving morons. Same with glass - banning it is better for everybody.