incrediBILL - 6:28 pm on May 7, 2012 (gmt 0)
Define "legally" as it's business, a matter of civil law, and it's a bit late to be concerned about contract terms after the fact isn't it?
Google created the contract, you agreed to abide by it, and them being able to cancel that contract at any time for any reason is a term you agreed to when you joined AdSense.
Besides, even if you don't like those terms, you're not in control. Your only alternative if you didn't like the terms was to simply not join AdSense.