What you're talking about is price discrimination
And yes the example you stated would be considered price discrimination because of product differentiation within the same market
Price discrimination is also used when the stores are geographically separated
It is considered as an unethical practice, however there are some (but very few) situations where price discrimination is allowed
EDIT:
Just realised how strange this situation was ...
I doubt the big guns such as KFC would share the same suppliers as other big guns in the same market
It's nonetheless still considered price discrimination. The supplier isn't allowed to give the business cheaper prices because it pays its bills on time, etc. There is no excuse.
As for the blame part, the supplier would get screwed, the ACCC would look into it and fines, imprisonment, etc. will ensue.