It looks like they're playing the iterated prisoner's dilemma. They have to compete in the same employee market over the long term. You snipe my engineer, I snipe your engineer, same net engineer count, but loss of experience. We both refrain, same net engineer count, no loss of experience. I wouldn't be shocked if this colluding policy hasn't indeed benefited the participating companies. And I could even see it primarily not being to keep salaries down, but to keep productivity up.

But it sucks for the tech workers. Not going to call me at my desk and try to get me to switch companies? Fine. But if I went to work for Google, it would suck if that put me on some kind of blacklist at all the other companies. It seems like they've crossed an ethical line here.