Sure, but the question is, where the line will be drawn.
A "Shopping" link next to "Maps" and "Images" is probably OK, but it's also completely useless. I'd be surprised if more than a fraction of people doing comparison shopping actually clicked that link.
Intentionally pushing down results of competitors via algorithmic criteria is obviously bad, but say they stopped doing that. What about the inline shopping box at the top of search results? That's still putting their own service in a prominent position. Probably bad...
... but then this concern immediately applies to translations, song lyrics, unit conversions, maps, shopping hours, etc. The kind of results that as a user I want to see. Will that be disallowed to? If so, this will make Google Search a significantly worse product.