Let's face it, not everybody has a good command of their own language. It isn't highly atypical to see somebody making huge mistakes in his mother tongue -from used words, to grammar. So which is the reason why everybody is allowed at this platform as teachers? I find it a real shame.
Edit: Okay, actually I wasn't referring myself to professional teachers, but community tutors. Sorry for the confusion.
italki is just a market place and all responsibility of choosing a teacher/tutor on the student. Unfortunately, italki doesn't do much effort to check the level of their tutors. And the situation when teacher/tutor can add other languages without being checked by italki is the wrong situation. I am studying English, so I will speak about it. Many learners face the situation when not native tutor has a low level of English. italki don't need too much effort to test it: just to oblige English tutors do the OOPT test, that is all. I don't know if there are similar tests for other languages, but I think that the English language market is the biggest here.
For this reason I am choosing native speakers from UK, US or Canada as teachers and tutors.
If the community tutor is really that bad, people should leave a comment. Eventually, market forces will dictate that the tutor will get less business. However, if they are charging a cheaper fee, then you pay for what you get. There is a market for cheaper tutors as well and you have to accept the limitations.
I sometimes make grammar mistakes in English even though I am a native English speaker and went through postgraduate education using English.