In your case it makes sense; you wrote the library at work. Google paid you to sit in Google's building using Google's computers and networks to write the library. If they want to own it, it's not unreasonable at all.
What is unreasonable is that if you did it at home, with no Google resources, that they would still ask you to assign them ownership.
(I, fortunately, don't have this problem. Part of the official open-sourcing process involves deleting any reference to our company, even in the copyright, so that nobody sues them if it breaks. Example internal app that was open-sourced: http://openefs.org/)
Why does my physical location make a difference? I'm free to go home as I please during the workday. If I duck out of work at 3 PM so I can go home and use my own laptop to work on a personal project, should it not be owned by Google? If I work on it in the mornings and come in at noon, should it not be owned by Google?
What is unreasonable is that if you did it at home, with no Google resources, that they would still ask you to assign them ownership.
(I, fortunately, don't have this problem. Part of the official open-sourcing process involves deleting any reference to our company, even in the copyright, so that nobody sues them if it breaks. Example internal app that was open-sourced: http://openefs.org/)