This has less to do with co-developing than custom-ordering, but it's the same strategy to me, that is, playing one OEM against another. It's just that a different company kowtows every year.
The Nexus 7 is a much more restrictive Android device than any other. According to Brian Klug of Anandtech, not only did Google omit microSD expansion (~$40 for 64GB card), they killed USB mass storage, too. USB-OTG is strictly for keyboards and such.
Therefore you're locked into this "tablet built for GooglePlay", in their own words.
It's like the smart-chip they put inside inkjet printers a few years ago to discourage third-party ink.
This is a business model where the device is subsidized by Google, rather than a free gift you seem to think.
According to Google's Rubin 'When it gets sold through the Play store there's no margin, it just basically gets (sold) through.'
Google hopes to sell you content and take your personal info to show ads to make money. Basically 'spoiled the market' for other vendors.