Dragons, are, Reptiles. Learning that many Dinosaurs had feathers is bad enough. The grandness of Dragons would be ruined if suddenly we found out they were/are lame as all Hell, Mammalian, freaks of nature like the Platypus.
Dragons arent Reptiles, nor are they mammals. Dragons do not nurse their young, as the definition of mammal dictates, and thusly are in an entirely different phylum all their own.
To explain the warm-blooded ness, a certain species of dragon (Official name: Frost dragon[
Draco Occidentalis Martimus], also called "Ice Dragons") migrates from [Ant]Artica. Cold blooded creatures could not possibly survive in both warm and deathly cold climates. Since dragons can cross-breed with each other, this explains the cross-special consistency.