Generalizing is not always useful.An example: there are some underdeveloped backwaters in China,such as Yuncheng of Shanxi,where FT's working for an IELTS company can teach with F visas.There,many of them are non-native speakers of English and most don't hold degrees from Western universities.Taiyuan Education Department won't stamp the documents authorising them to get Foreign Experts' certificates.So they can't get Z visas for anywhere in Shanxi.But locally in Yuncheng they can get F's.Do you want them to return to Africa while the middle school students there do without an FT?They'd have to do without because the vacancies couldn't be filled by FT's with Western university degrees who can get Z's there.Why?Because too few of them apply to there.
Now,FT's with Z's have sometimes been cheated and badly treated.But with a Z you might hit trouble at the airport if you do a bunk.In contrast,if you do a runner with an F,nothing will happen to you.
As for being deported,you're wrong.The wording on the * visa is 'you are requested to leave China within 10 days of the issue date'.That's not deportation,which is being forced to go back to your homeland!You can go to South Korea for a holiday,for example,and then re-enter on a new L or even a new Z if you act fast,issued by the Chinese visa office in Seoul.Geberally speaking,it's advisable for FT's to teach with Z visas and Residents' Permits.but for those with inferior paper qualifications or from countries whose first language is not English,teaching with an F is better than not teaching at all.Let's consider all kinds of budding FT's,not just native speakers of English with Western four-year degrees.
I can assure you that a couple of years ago in Yuncheng there were at least 18 FT's teaching with F's,and not all of them were from Africa!