Because they have TRADE SECRETS to protect. Secretes which are both theirs and ones that they have licensed and contractually are bound to protect.
I don't think they are anti-open source, they are just trying to protect their intellectual property. They are still releasing drivers for these devices and although you may not be entitled to see the source, you can still use that open source operating system with that shiny new video card.
I keep receiving mailouts which suggest that US patent rules have changed in recent years such that keeping trade secrets is an increasingly advisable business strategy, instead of acquiring patents.
I don't know if that's true, but it could be part of what's going on.