that case is made out of steel, and steel is capable of rusting. however, this is why steel cases are painted and generally they are painted with an electrostatic method using good quality baked on paints which bonds well and is good for wear and corrosion resistance.
likely those two brackets were not properly painted or prepared for painting which caused them to rust. if they are the only components rusting in your case than this would make the most sense that they are some sort of defect.
if the case is in warranty you could likely get replacement brackets mailed out for free as this looks like a defect. otherwise, they would need to be sanded down , cleaned and painted. a dark grey paint with metallic fleck like that should be...