So, this kind of thing happens all the time, and there's the Public Suffix List for exactly this problem.

There would be no issue at all if Verisign, or maybe Global Name Registry, decided to stick to the 3rd level registrations exclusively. Problem is, the chucklefucks over there decided it was a good idea to also hand out 2nd level registrations. Those 2nd level registrations outnumber the 3rd level registrations by an order of magnitude, so the PSL decided to just let joe.smith.name and john.smith.name share cookies. Which, IMO, was not a good decision, but it is what it is.