I'd be highly surprised if Apple took such a step - such an acquisition, or major investment, wouldn't bring Apple any greater profit, in exchange for billions. Given Apple's tendency to make any purchases as small as possible, I suspect we've seen all of their involvement - some fee in exchange for the unhindered embedding of Twitter posting within iOS and Mountain Lion.

To me, the most likely reason which explains the "consistent user experience" quote is simply the matter of presenting ads, and being certain that every single user is seeing them, unfiltered.