Most ToS state that their terms are subject to change. I do not know whether or not a company has to explicitly state that the terms provided are perpetual, but there are lots of examples of companies changing their ToS retroactively.

For example:

http://arstechnica.com/tech-policy/2008/09/google-on-chrome-...

http://boardingarea.com/viewfromthewing/2012/10/10/united-ma...

And here is an interesting paper about retroactive lawmaking by the government:

http://www.law.harvard.edu/students/orgs/jlpp/Vol30_No2_Weie...

Pretty much all ToS have a clause that states that the terms can change any time. Where services differ is whether they'll provide you with advance notice (or if you're expected to check for changes yourself!), and if you are able to close your account and have your data removed (I believe this is required in the EU).

Looking at http://www.whatsapp.com/legal/, it appears that WhatsApp falls into the "we can change our terms without letting you know" category, and I don't see anything about their data retention policy :-(

I am quite certain that they are not allowed to retroactively change the ToS and access the data legaly.

That doesn't mean that they won't. Perhaps they will just do it behind the curtains, offering better Facebook Friend links.