I was in Yellowstone last June and don't recall any specific knife regulations. I carried a Mora, sheathed, on my belt half in my pocket almost all the time, never had any problems with any rangers or in the camp store. In fact, the camp store sold knives, including some Kershaws and Bucks. Nobody seemed concerned with my use of a machete to split firewood.
I believe they have a generic "no weapons" policy (as most national parks seem to have) that seemed to mean more "no loaded firearms" than "no knife". I don't recall the specifics of the policy, however.