So, why does the Bing app have a search bar in the app rather than relying on the charms bar? You'd think that if this was really a guideline that at least the built-in apps should follow it.
Probably because the Bing app was written by the Microsoft team responsible for Bing, rather than the Microsoft team responsible for coming up with the guidelines.
It's hard to be consistent with guidelines that are in flux for a feature that's being developed by another team sitting in a different part of the city when you have your own timelines to complete the work.
For the record - this isn't an excuse, just a reason.