Friends often shove and shoulder-punch each other without it being self-defence. It's usually not even considered "hitting". But accidents do happen. Especially when substances affect people's sense of balance and floors are uneven.
Here's a much more common way to go to prison for something silly: You have a baggie of weed in your car. You roll through a stop sign. Get pulled over. Cop smells the weed. You go to jail. Oh, you just bought plenty so you can share with friends (they all pitched in). Bam, charged with intent to sell. Serious jail time.
It's really easy to go to prison for stupid stuff.[1]
In fact, on average everyone commits 3 felonies per day. --> http://www.wsj.com/articles/SB100014240527487044715045744389...
[1] Hell, as a teenager I got arrested for shoplifting something like 20 euro worth of candy. No jail, but talk about stupid reasons to get arrested.
Please tell me the situation you described putted 2 M people into jail! You must have the highest "accidental death" rate of the universe.