[1] https://lists.swift.org/pipermail/swift-evolution/Week-of-Mo...
https://en.wikipedia.org/wiki/High-Tech_Employee_Antitrust_L...
a lot of companies have non-solications in place to try and prevent this kind of behavior
If you know of other companies engaging in backroom nonsolicitation agreements with each other, the responsible thing to do for your fellow STEM workers is to call the Department of Justice Antitrust Division and let them know that the Apple/Google/etc. settlement didn't achieve its intended effect.
In practice, what most big companies do is they ask incoming employees for the names and personal contact info (and only that) of good employees that they have worked with in the past, and then pass that info on to HR, for a corporate recruiter to make the initial contact. Has the same effect, but no contracts are broken, since the corporation does all the soliciting.
(just read the 2nd paragraph. the rest of the article makes the rather obvious point that, if you solicit based on confidential company information, 16600 won't save you.)
[1] http://www.pashalaw.com/legally-poaching-employees-company-a...
[2] http://www.nolo.com/legal-encyclopedia/understanding-nonsoli...
IANAL but I did some research on this and to the best of my understanding these are perfectly legal and enforceable in California.