I've always been under the impression that the norms in Silicon Valley are often different from the rest of the country, so that's not surprising. My experience is limited to the North-East/East-Coast though, so it's possible that this is a more regional thing, or possibly even specific to the industries I work in.

It's possible that state-level laws come into play here too. For example the fact that non-competes are not enforceable in California, where they are generally enforceable on the East-Coast could be a factor in the kind of relationships companies favor.