I suspect they're just being cautious with the legal side this early on. One reason to be cautious about the patents under the pledge: there might be some grey area in the language where say, a competitor could make hostile moves against Google while still falling under the protection of the pledge via shell companies.
Side note: isn't this already their internal policy? When has Google ever gone after open source projects with any of their patents?
They never have gone after FOSS projects, which makes it all the more strange that they haven't been willing put all their patents under this. Presumably they have no outstanding suits that would be affected.
I don't know. They're trying to create a legal contract that binds them and stays valid as long as the duration of the patent. I'd be pretty careful about putting every patent available under that agreement until I was sure that it couldn't be used against me in an unintended way.
Side note: isn't this already their internal policy? When has Google ever gone after open source projects with any of their patents?