Good analogy, but it's no secret. Google is more than happy to take your open source, contributed works and use them for profit. It's free engineering work on components that just aren't very useful without all the proprietary other parts, and as long as there are corporate fanboys out there willing to do it, Google and Apple would be stupid to not take advantage of it.
I am a little disturbed that I cannot find reference to any way that the site owner can "opt out" of having a sidewiki hooked to their pages. At least with Microsoft SmartTags, there was a way to disable them with a meta tag in the html header, and unlike Microsoft, Google has enough geek fanboys who think Google shits gold out there to make this feature take off.
I used to have comments enabled on my Flickr photos, but jokers kept on leaving suggestive remarks about my wife (she's pretty hot, IMHO). So, I turned it off. When talking about this with a colleague yesterday, we came up with the "ugly kid" scenario:
Imagine you have a family site with pictures of your kids on it and some jerk writes, "man, you have ugly kids" on the sidewiki. What do you do? You can't remove it. Will it be filtered out automatically by Google with their so-called "quality algorithm"? Just because there will be no anonymous posts, don't think that people won't do things like this.
Seriously, has anyone seen anything about a way to turn this off for your site? I'm not against free speech and all that, just don't add it to *my* content without my permission. Whether sidewiki is considered part of the page content is academic: the visitor will see it attached to your page.
Suggest you just sit there and wait till life gets easier.