Ya know what? Airport traffic areas are within 5nm of the airport, with some extensions. If you plan your activity within that 5nm area then you are required to notify air traffic control or base ops or flight facility responsible for that area. HMMMM C'mon...The 400ft limit is just added safety. The lowest allowable VFR traffic pattern is 800ft above ground level(normally, generally and as a rule). That gives us 400ft of verticle separation. Full scale aircraft is 500ft separation. Who flies above a couple of hundred of feet anyway?
I'm with PIGG, here comes the horse puckey. 400 ft this and 500ft that and omg this and that....

Okay, now after posting this, maybe I'll read the whole thread....(this is the edited part.)