Entirely justified, though regrettable. The kinds of things they ship our way are usually in the small-operator area that's always been a problem for us.
The green area is where most developers (though not all, which is where things get complicated) reside, and those are manifestly off-topic. Some are in the purple area and do make good questions, but typically fail the "development systems are off-topic" test. It's these kinds of systems questions that get sent our way instead of SuperUser or U&L.
And the rest of our inbound migrations? The normal green-area stuff of
Oh, this looks like a bunch of computer nerds, I'll ask my question on this site that has a lot of traffic on it
...that we get daily, but SO gets more of as they're a much bigger collection of computer nerds. The large majority of which are home or hobby systems, as we all know.
Overall, systemically, the SF migration target worked w-a-y back when it was just the trilogy. Time has passed and it is now manifestly clear that the StackOverflow VTC-base does not understand the nuance of our scope. It is being abused, and creating a lot of work for us. It should be removed.
And I'm very curious how SuperUser's SO-reject-rate has shifted now that we have that nifty review-queue thingy. Before it went in, they were rejecting fewer than us but still well into the teens. SU and SF may have a similar problem.
It may be a better idea overall to replace the SF and SU migration targets with U&L and DBA, though that's the kind of thing that should be really worked out between all of the communities involved. The only place on the entire StackExchange where it's safe to ask generic "how do I set up my dev-system server binaries" is on SU right now, so that may not work.