It is not only for starter but for the whole platform ecosystem. It is not even user-hostile. It is just to prevent "Nah forget Firefox add-on market, just install this file" fragmentation. Firefox add-on platform is already much smaller than Google chrome (of course). I wouldn't be happy if the market got even smaller because of the fragmentation.
And Firefox already allows us to install whatever we want in dev/nightly version, for hackers. So I don't see any problems here.
No, just a requirement that you submit your addon for signing, submit to a separate agreement[1], and their policies[2] (which has separate sub-politics about data usage), regardless of whether you offer it on AMO or not.
What part of behavior is user-hostile?