The FAA and congress have repeatedly made it clear that a "Model Airplane" can not legally fly above 400' agl anywhere in the USA. The fact that it is unenforceable does not make it invalid. Get spotted by a full scale airplane at 1000' and get caught, then see what happens.
No they haven't, there is nothing documented anywhere that makes such a flat statement. However, if you are caught invadeing teh NAS and creating a nuisance or danger, they will hang you by the balls.
It is interesting that FAA is proposing that a commercial UAS under 55 pounds can fly up to 500' where flight is not prohibited.
And, your point being?