It seems like there are some people who downvote before even reading the posts, already assuming that the person is guilty.
And you can tell that because they make some generic response like "if you get banned you deserve it" and some of them are not even related to what the OP is actually saying, so it becomes painfully obvious he didn't even read it.
A few people are like that, sure.
But the vast, vast majority of people that complain about their punishments on these boards demonstrate clear violations of Riot's rules, so their complaints come across and childish and time-wasting. Weak rationalizing of their behavior, bargaining, pleading, etc. all result in meritless "discussion" threads. I personally don't go around downvoting each of them, but I can understand those that do.
Honestly, show me one example of somebody punished for behavioral issues that, by Riot's rules, didn't deserve it. I don't believe I've ever seen somebody that produced exonerating chat logs.
Normally the reasons why I downvote:
Want an example of why people here are so cynical? Check out this fuckstick:
He actually posted a previous thread that went un-noticed, claiming innocence and offering evidence. I, personally, suggested he re-post his plea with a clickbait title and get some attention. The above thread has 176 upvotes (at at one point had more, if I recall correctly). It got the attention of a Rioter, who immediately realized that the reform card had simply glitched and the actual chat logs were incredibly toxic. OP didn't even slightly play ignorant after that. He laughed, spammed some 
at me, and vanished. Just another successful troll.
Well, depending on the behavior some of them have already been proven guilty (such as the use of 3rd party programs.)
As for the others, I couldn't tell you however I can speculate that they probably feel if the person was actually innocent then they wouldn't be posting on the boards about it since that has no impact on their punishement. The correct way to dispute a punishment is to submit a ticket to Riot.
this question is related to human decision making
which is unpredictable
There's a lot of reasons, most of them not worth peoples' time.
The auto-downvote happens in all subs, not just PB&M.
Sometimes it's because the OP obviously leaves out key info to curry the community to their side and it backfires (since they've seen such tactics before), sometimes it's because they don't agree with what the OP is writing in the title of the post so they want to disagree but don't feel the need to post.
I've seen a few posts with fairly innocent chat logs but the majority falls under one or multiple of the categories above.
False bans are almost non-existant. We used to believe people were innocent but after years of being proven wrong it's not worth believing someone didn't deserve the ban.
Those are likely the toxic troll baiters that draw people into bans in the first place.
Because almost every overturn my ban thread is made by an extremely toxic player who deserved the ban
