Google cares in as much as it helps facilitate free labor for open source projects they heavily use themselves. I would venture to guess if its critical software to google that google group is going to be nice and tidy and free of spam.

I honestly didn't know anyone still actively used Usenet. I thought Google Groups was quite valuable as a tool for reading historical data from decades past, now that most ISPs don't include Usenet access. I wouldn't have expected spam blocking.