I bought the game in 2011 (iirc), I guess they could justify this with the "Other" section of the terms of use [1]:
"We reserve the right to change this agreement at any time with or without notice, with immediate and/or retroactive effect."
No idea if this is enforceable in the US/EU.
[1]: https://web.archive.org/web/20111002140457/http://www.minecr...