In a different tweet the author said that it was for public intoxication. Austin is a big college town and throwing young guys into the drunk tank for public intoxication is a pretty common thing in these type of towns. And I am willing to bet that most people who are thrown into the drunk tank think they "did nothing wrong".
A reasonable explanation is this guy got a little tipsy and the police thought he was drunker than he was, threw him into a drunk tank to sober up, put a spit mask over his head in case (or because) he was being belligerent or he might puke, then released him once he was clearly sober.
Don't get me wrong, I don't think this is fair treatment. But for anyone who has spent years living in a college town populated by tens of thousands of drunk 20 somethings, it is at least understandable why the citizenry would want police to respond this way. It is also far from a sign that the US is racists as a whole or an (extra) reason we should be afraid of Trump which the author seemed to suggest in other tweets.