Most planes with flat bottom wings happen to be trainers with designed in self correcting tendencys but I don't believe it is the flat bottom wing that gives them those self correcting tendencys. Dihedral, tail moment, center of mass location has a lot more to do with it. Oh yea, let's not forget wing loading. I'm pretty sure a symetrical wing trainer could be built. Take away the dihedral and move the CG back a little and voila, your flat bottom winged trainer is no longer self correcting.

It's like assuming that cigarette smoking causes crime just because the majority of prison inmates smoke.