Entity riders and their passengers can't be pushed by other entities and have no collision

When summoning entities with a passenger entity then the root entities won't collide with other root entities, and the passengers won't collide with other passengers. Entities pass through one another.
I expected entities to push/collide with each other.

EDIT: Noticed it's still in previous versions like 1.10.2 and 1.9.4, might be related to MC-15202 as in the fix for that one caused this.
EDIT2: Checked to make sure and the issue happens since 14w27a, it's not in the previous snapshot 14w26c, so that would actually point directly at changes done to fix MC-15202

Reproduce with these commands:

/summon minecraft:chicken ~ ~ ~ {Passengers:[{id:"cow"}]}
/summon minecraft:cow ~ ~ ~ {Passengers:[{id:"chicken"}]}