I asked about something similar at the offer stage: invention disclosure and assignment. I asked several times and the recruiting team told me that "everything I needed was on the website [where the job offer and the related terms were]." I even asked "No, I mean the forms you'd ask me to sign upon accepting the offer when I'm hired." Still nothing.
Sure enough, as soon as I accept the offer I'm presented with exactly what I was looking for on that very same website.
Somehow I think "never attribute to malice ..." doesn't apply to corporations.
I can't speak for this case, but the answer is often "you found out after you travelled across the country and turned down other job offers, and may be waiting for repayment of moving expenses".
Yes, a person could try to get this sort of thing in writing and sue for damages incurred, but they'd still be unemployed and loaded with expenses in the meantime. There's a lot of freedom to push this kind of thing because there's too much legal overhead to fight it.
Perhaps I should have. I was assuaged by the fact that they provided specific "exceptions from copyright" (e.g. open source projects that I have and will continue to contribute to).
Sure enough, as soon as I accept the offer I'm presented with exactly what I was looking for on that very same website.
Somehow I think "never attribute to malice ..." doesn't apply to corporations.