I imagine they have an undocumented api for reverse image search. The team at Google has released this extension[1] for adding reverse image search to the context menu, which is currently the only extension that enables reverse image search for images only accessible while logged in (think if you want to reverse image search a preview of an image someone emailed to you).

I was thinking about intercepting the requests it makes and making an open source extension that does the same thing, but I haven't had the time yet.

[1] https://chrome.google.com/webstore/detail/search-by-image-by...