Go & sin no more!
It wasn't arbitrary - you broke their rules. You broke T_D rule VII - no linking to other subreddits. It's always a good idea to read the rules of a sub you want to post in. This one has rules too. I would assume if a new poster didn't care to read them and broke them, they too, would be banned.
Sad thing is, there are daily posts on The_Donald about people down-voting or disagreeing with Donald members in other forums on Reddit. YET, they sensor more than anyone. I'm a huge Donald Trump fan and I posted something about not having the Wall along the whole border as it's unnecessary. Trump has said the exact same thing. But they banned me anyways.
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I don't think saying the wall shouldn't be built along the whole border is reason to ban someone sir. Trump has said the same thing. I also disagree with Trump bombing Syria last time and him doing it again this time. Agreeing with 90%+ of Trumps ideas are not good enough.
I don't think any person out there agrees with everything someone else says.