set render border bug ? or normal?

Mac OSX : 10.8.5
Blender : 2.75.2 latest build bot

my set render border at input editor

Q> id like to know if this is normal or Bug
if i want to render only camera inside box to reduce memory consumption during raytracing or quick view
when i large the marquee selection … it will not detect the camera box and also raytrace the outside camera

as my knowledge it should detect the border of camera and automatically render the inside box even i do large marquee camera border render

thank you for answers

[SUB]i find alternative tool
Amaranth Toolset - camera as render border