At most Canadian universities, for physics (not sure about math), we have a similar GRE requirement. It is only recommended for Canadians, and required for international students. I talked to a professor about this, and he said that they do this because they are not familiar with the physics programs in other countries. They don't know how good your institution is and being in top 5% at a bad institution doesn't mean anything. The GRE score gives them an indication of your skill without having to know where you are from. I wouldn't be surprised if Cornell and Brown did the same thing. Have you thought about emailing the universities about how "recommended" the GRE is for international students?