Yes, but... With a sufficiently widespread third party library they risk a backlash with their userbase. Social networks depend so much on the network effect to bring in users that cutting out a large chunk of users all at once because the protocol changed could cause more users to drop out.
Thinking about it, I wonder how much AIM and MSN Messenger's fights against third party clients messed up their user bases.
Their userbase uses the Whatsapp app that comes bundled with their phones on many intl carriers. I don't think they care about the 0.0001% of their userbase that uses third party clients.
Presently, correct. But that's the risk of allowing third-party clients with an unpublished protocol spec. Right now they can break anything they want. If they don't limit third-party clients, their hands could become tied by too many people using it.
EDIT: Note: I'm not a fan of proprietary protocols. I'm just describing what I see as the position of a company that wants to monetize a network like this. If the network and client is the revenue source, then third party clients work against you. Allowing the third party clients to gain too large a share of your user base means that breaking compatibility could have significant network effects against you as those users move to another platform and bring their friends and family along.
I don't think there ever was a serious risk for that for WhatsApp: multi-device support isn't just missing, they are actively making it a pain. There's no way to obtain your password, the password changes regularily, logging in with a second client kicks the old connection, etc.
So people would be forced to make a choice: use it on your phone, or on your computer. Aside from the group of people who don't have a smartphone, most people would chose phone.
MSN made a ton of protocol changes even after there were a lot of third party clients. Trillian, for example, was very popular. Trillian had updates out for MSN changes typically within a day or two.
