In your OP you mention "At these airports, it is very easy to walk from a domestic gate to an international gate", when also referring to YYZ, which is also not true.

To get from Domestic (D) to International (F), you need to pass through two checkpoints - the first when you cross over over the D gates, where your BP is stamped and passport inspected to allow you to walk down the long corridor to F gates.

At this point you're mixing with some of the E (Transborder) arrivals, so when you approach the customs/international split, once again your passport and BP are inspected and the agent has to individually unlock a security door for each passenger.

With those double checks, I doubt a domestic passenger can easily enter the International section. That's also one-way; to leave International you'd need to circle back around towards immigration, and could only re-enter International a another security checkpoint or go through CDN/USA immigration.

Note: It is possible to re-enter YYZ bypassing immigration, but only if you prove you were either a domestic connection or checked in at YYZ, and is hit and miss.

So International is sterile at YYZ, although to your point about needing to go through a small CBSA checkpoint to re-enter international-to-international, you are correct. They would never dump passengers back into the F gates, for the reasons canadiancow mentioned.