I agree, but Google doesn't just get rid of prefixes.

news.ycombinator.www.com would get normalized to news.ycombinator.com (not sure what would happen when using the www tld)

That's an extreme example, but there's an obvious security risk. This is a half assed change with buggy behaviour, regardless of whether you think www.domain.tld is materially different from domain.tld or not.