Because if he only translate it and post the translated text its not a plagiarism in my eyes !
That's correct if he uses his own words to redact the post. But if he use an automatic translator and post it as it is word by word then it is plagiarism.
It is just another trick to avoid getting caught.
You are misunderstanding what plagiarism is.
Even if the user was to rewrite it completely it would still be plagiarism. It is just harder to detect.Also even google translate does better than translating word by word now. It doesn't make it any less plagiarism.
These people can be tagged/banned for plagiarism and/or using automated translation tools to gain visibility/activity/whatever.
rewrite it completely is not quite the same as translate it completely. I would think a rewrite makes it not plagiarism while an accurate translation makes it plagiarism. A rewrite means more change then a translation does. But I suppose it is a matter of semantics.