Rust at rear hub

My rear hub/axle/bearing area is getting pretty rusty. There is no dust cap like in the front.

This may be expected, but is cause for concern? For example, if I wanted to remove the rear rotor for the RLCA bearing mod, I can imagine it being rusted on pretty hard there.

Why shouldn't we run dust caps on the rear axle?

The rust is coming from your brake rotors.
I actually had some pooling of rust on my wheels after heavy rain.
I had the same problem. Swapped out some zinc coated Brake motive discs.
Problem solved.