Don't understand MOSFET as amplifier
It is an amplifier because you put a wiggly signal in, and you get a bigger wiggly signal out.  The fact that the output wiggles down when the input wiggles up is a trivial detail, that -- if it's a problem at all -- can be solved in any number of ways (not least of which is following one stage by another, because two negative gains, when multiplied, result in a positive gain).