if a steel frame hasn't been painted on the outside or coated on the inside in some way it will rust. the nice thing about rust on the inside is that the rust is well protected from the elements. and that rust can somewhat protect the inside from more rust, like titanium oxide protects the outside of titanium frames if not disturbed too much. it's just doesn't protect as well as titanium oxide or aluminum oxide. of course if water is forced into the frame or allowed to drain into it and disturbs the rust, new rust will be created and over time the inside of the frame will disintegrate.

if i were planning on washing my bike with a hose and did not want take the trouble to avoid water penetration points like the headset, bottom bracket, water bottle holes, chainstay holes, etc. i might look into having the inside of the frame coated with some sort of rust-preventative, as mentioned.