The number one difficulty in every police action and every charge laid is when you appear in court you have to identify the accused individual. If someone is masked, generally speaking, their intent is to escape that identification. This happens all the time. On a normal basis someone runs away into the darkness, hoping to evade identification. In a riot situation, then, you have a large group of people. The only way to evade that identification is to wear a mask. The intent is the difficult part in the whole situation. That's what we have to prove under existing legislation: that the person simply wasn't just standing and wearing a mask but actually had the intention to continue and commit a criminal offence at that time.
I can't speak to the Vancouver situation. I have seen photographs, and I could imagine the difficulty in trying to identify groups of literally hundreds of people who are all masked and committing offences.