Interesting, as I never gave this much thought. However, I must think the same, as I do not have Treasure of the Sierra Madre in my western genre section (I place my discs on shelves grouped by genre).
Unforgiven is a terrific film -- I would highly recommend it. Other favorite "non-spaghetti" Eastwood westerns of mine include The Outlaw Josey Wales, High Plains Drifter and Pale Rider (his remake of Shane). Josey Wales is one of my very favorite westerns.
For anyone interested, I can provide...
I've never been a huge fan of Shane, either. I saw the movie for the first time in a college film class in the ealry '80's and was instantly annoyed by the kid Joey. Out of all the films which are considered "great" westerns, it's probably my least favorite. I never bothered to upgrade my SD-DVD...