Why the register name is not keyword(color)?

what if use the register name to be a variable accidentally?

int register = 0;

will not compile.

Arduino: 1.6.6 (Windows 8), Board: "Arduino/Genuino Uno"
...
...
sketch_may06b:1: error: expected unqualified-id before '=' token

 int register = 10;

              ^

exit status 1
expected unqualified-id before '=' token

Ok, thanks.