intersection, union, difference, symmetric difference for dictionaries

[mauro]

 Dictionaries and sets share a few properties:
- Dictionaries keys are unique as well as sets items
- Dictionaries and sets are both unordered
- Dictionaries and sets are both accessed by key
- Dictionaries and sets are both mutables

So I wonder why operations such us intersection, union, difference, 
symmetric difference that are available for sets and are not available 
for dictionaries without going via key dictviews.