They also have native apps on most mobile platforms (including Windows Phone, Blackberry and even Nokia Series 40). It isn't as easy to update all of them as one would think.
But those are the apps themselves. This is about the WhatsApp Web protocol which allows you to use WhatsApp from a browser window by pairing it with a live session on your phone. Since the API endpoint are their own servers and the client is literally just a website, they can update whenever they want however much they want.
One of the things that has repeatedly pushed me off WhatsApp has been their very aggressive stance on this: I've several times stopped using it because "you need to update your app or this will all stop working in 7 days" and I've not been in the mood for jiggling the apps round on my tiny phone to be able to perform that update: the 7 days has passed, and I'm off WhatsApp again until such time as I "need" it.
