English Original Sentence
No wonder he was arrested.
I had expected the subjunctive, arrestaran. 
The phrase “no wonder…” is like “that explains why…”
So the person being arrested in the sentence was an actual event that was completed, despite the word “wonder”.
I see your point, but I found this thread about no es de extrañar que and choice of mode:
Also, I think one can see this expression as another way to say es lógico que, which does take the subjunctive:
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English Translation
No wonder he was arrested.
The sentence on Tatoeba has now been changed to say arrestaran. This was in response to a question raised by @morbrorper.
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