Without a doubt a chin bar can be grabbed. But in some of those situations a cop can head butt with that hard helmet while taking other defensive action.
For every time someone might grab the chin bar, there are more situations when the helmet can deflect things thrown at an officer, protect from blows etc etc. So it's far more a protection than a liability as others have pointed out.
Some of the posted photos show Euro cops have long worn better gear. Would be interesting to see what they wear in Spain and Southern France and Italy ...where it's hot. (Well... I was in Southern Spain, in May 2007, and two motor cops were in the air conditioned gas station cooling off. They had full (Gore Tex or similar?) protective suits and flip up helmets. Who knows if they continue to wear em in June and August. I bet they do. If them fluffy Euro cops can tolerate heavy riding gear I bet rough tough American cops can!
Foot pursuit in riding gear is obviously a consideration. There must be some police conferences where discussions on motor cops ditching their bulky protective gear have been presented?
Like a demo on how fast Spanish cops can get out of an aerostitch while unholstering his taser....
In theory some (pricey) cool suits could be plugged in under an Aerostitch. Wouldn't help for long off the bike. But I guess that's not too likely as even basic protective gear seems forbidden.
Even in 1978 German cops and CHP wore full leathers and flip up helmets or full face helmets. Rode in sun and rain. Now I see from that photo that they have gore tex or similar. Darker green now. The old green was more visible.
Still even with all those considerations, something better should be done for the most exposed cops on the beat, eh? I hope it happens.
Thanks for all the good work ya'll do.