I was just thinking of a similar example... see below.
I really do appreciate you libertarian philosophy in most situations here. I feel an exception to that is when the safety of others is imperative to the focus of the employee. For example, ~5 years ago in Boston we had an incident involving a train conductor distracted by his/her cell phone and rear-ending another train (I believe there have been others as well with buses, perhaps with other trains too). That resulted in the injury of passengers. Now, conductors (and all MBTA vehicle operators) are now banned from possessing cell phones onboard during their shifts. Without a ban on an item, such a cell phone in this case, you help eliminate the likelihood of a plausible accident. Will it prevent all such accidents? Maybe not. Will it lessen the risk? Probably, more so than the alternative.
There is the possibility a bus driver might keep their phone in their pocket and use it in a responsible fashion, such as after their shift. But I'd rather see the sacrifice of that liberty than see a bus collide with a train, killing and injuring dozens. That one time the driver decides to check his/her phone while driving could be the only time.