I'd think it's because the NETWORKDAYS function, prior to XL07, exists
in the Analysis Toolpak Add-in, which is an "other...workbook".

In article ,
Suddes wrote:

When using conditional formating i get the following message when trying to
use NETWORKDAYS' function

"you may not use references to other worksheets or workbooks for conditional
formatting criteria"
! any ideas why this would be the case

many thanks