Reddit is heavily astroturfed by bots, corporate advertisers, and intel agencies. On the default subs/main pages, I seriously doubt that the majority of posts are even from real people. Not sure reddit opinion is representative of anything, but then again plenty of people will trust whatever they see appearing on TV, from politicians, or their social network feed.
It's pretty hard to get a general read on the public these days, and like you point out there are plenty of people that have no real principles and take glee seeing the police being used against their political enemies.
True about the astroturfing... and of course the mods of major subreddits are part of the rot.
Sadly my experience is that real people do hold these shocking opinions as well. Hopefully you're right, and reddit badly misrepresents the proportions.
There definitely are real people on there too, but they're mostly teenagers and the kind of people who stay home on the internet all day. So take solace in that even if all the posts and comments were from real people, the opinions and beliefs on there aren't representative at all of the general population.
Imagine we ask reddit, seniors in a Florida nursing home, truck drivers, banking CEOs, university professors, high school students, inmates in prison, or soldiers what they think about police. I'd expect wildly different answers from each that don't give me insight into what the "general" opinion is.
Police shut down public transport, and set up roadblocks surrounding the CBD to stop anyone getting to the protest in the first place. As far as I can tell, these protesters just wished to walk around and protest peacefully, but weren't happy to be impeded.
Some unnecessary behavior by the police documented, but many Aussies seem supportive of police violence because they disagree with the premise of the protest (see reddit comments).