Are these Markets safe? They are getting access to some my credentials, they can distribute something on behalf of my name, like virus, or spam, etc.
If they ask for permission to distribute your apps, then they're probably the least to worry about; you can always verify that your apk is being distributed unmodified by verifying the apk's signature with your public key. You can also publish your public key (but keep your private key private) so that people downloading from these alternative markets can verify for themselves that the apk came from you unmodified.
Would it be fair for Google to host apps there? I mean Amazon is ok, that's fair competition, but I'm afraid these "noname" alternative markets are somehow using unfair benefit of the work of other
It would be just as fair as Amazon; in fact, it would generally be considered unfair monopoly if Google started hunting down these alternative markets. In any case, the Play Market does not provide paid apps in China (and many other countries), so it does not hurt their revenue stream.