It's more than "they all do that"...it's a matter of they all should do that. It's how the charging system is designed to work. If the GEN light doesn't come on with the ignition on or doesn't go out as the rpm rises above idle, then there's something wrong.
Yours, or now his, sounds right to me.
The usual question from people new to an airhead is along the lines of misinterpreting the GEN light idle glow as the oil light coming on.
Knowing how it should work is important as the signs will be there to make an early diagnosis of a charging problem.