Why the Int'l boarding pass checks at YUL
 

I'm not asking what they are or how exiting through customs after going through them works.

But a few years ago they weren't seen as necessary even though in theory the same issues with duty free stores and customs existed then. I don't believe any customs regulations changed in any relevant way and yet suddenly the airport was happy to install this annoyance for its users (and also reduce their flexibility with gates and the value of their lounge leases).

And there are other airports that don't have these checks -- I was recently in YYC for a domestic connection and they told me to walk over to the international area to use the lounge there. There was even a place where you have to pass through a passage to get there where it looks like it was intended to have them. But they don't have them and it benefits travellers and the airport.

So what changed a few years ago and what compelling argument convinced YUL to do this to us? Does it benefit anyone?

What makes you think the airport was happy to install these gates, as opposed to being forced by the authorities?

As far as I know, the gates mostly prevent people inside the intl. departures concourse from walking back out into the domestic section and then out into Canada. This, in turn, allows intl. to intl. transits at YUL for people who have not gone through Canadian immigration.

It's this. YYC used to offer sterile transit connections, but no longer does. (See the wiki post for more details.)

I'd imagine the O&D that uses this most must be FSP <==> PAR.

While I don't have stats to back it up, I am almost certain that you'd be wrong. The passenger volume to/from FSP is tiny. On the other hand, AC does roaring trade selling connecting itineraries between Europe and the US as well as the Caribbean via its Canadian hubs, including YUL.

Don't ITI itineraries have to go through a very minimal and quick CBSA check? I wouldn't know, because the two times I've done ITI at YUL were during major Christmas disruptions where the CBSA officer I was expecting to meet was replaced by a red box where you deposit some form (I forget which form I filled out) before proceeding to my international gate. I posted a picture of the red box in some AC forum thread a couple of years back.

I haven't done ITI at other Canadian-eligible airports but I imagine that setup would be similar...

Yes exactly. But you don't have to be admissible, and are not admitted, to Canada.