[stingraycharles]

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...