[idn] Question about a ToUnicode step

[<jim@mathies.com>]

Hello,

I'm trying to understand the purpose behind some steps in 
ToUnicode. From the IDNA spec:


1. If all code points in the sequence are in the ASCII range (0..7F)
   then skip to step 3.

2. Perform the steps specified in [NAMEPREP] and fail if there is an
   error. (If step 3 of ToAscii is also performed here, it will not
   affect the overall behavior of ToUnicode, but it is not
   necessary.) The AllowUnassigned flag is used in [NAMEPREP].

3. Verify that the sequence begins with the ACE prefix, and save a
   copy of the sequence.


I'm curious about steps 1 & 2. I don't understand why nameprep
is being applied to ASCII domain labels. This seems to be pointless 
in situations where the input string is 8-bit ASCII. Wouldn't
a simple dns character compatibility check in place of steps 1 
and 2 suffice?

Regards,
Jim Mathies