Google+, the Play Store and many other services are not open. In some cases, they may have APIs, but they are not open APIs. They have closed down more open platforms, such as Reader, in favor of the more closed platforms, like Google+.
That doesn't mean they are a closed company today, but I believe they've turned the corner. Just like Facebook has a lot of OSS projects but isn't an "open company", I think Google has shifted the balance away from openness, even though they have a lot of OSS projects. I see them as qualitatively the same, but significantly different in degree. I wouldn't have said that three years ago.
EDIT: I realized I answered how Google is closing, but not how they are closing the web.
I think that has to do with two things: preferential treatment of their services in search results and hiding information behind Google logins. The former makes it harder to find anything Google hasn't sanctioned; the latter makes it harder to go elsewhere to get your search results.
I see it as exactly the same thing as Facebook pages for businesses instead of web sites. Sure, http is fine and intact, but now we are beholden to Facebook for information.