But the rule doesnt say "politics" are forbidden in the grounds...
Here is the rule:
"Engaging in any act that, in Blizzard’s sole discretion, brings you into public disrepute, offends a portion or group of the public, or otherwise damages Blizzard image will result in removal from Grandmasters and reduction of the player’s prize total to $0 USD, in addition to other remedies which may be provided for under the Handbook and Blizzard’s Website Terms."
Is up to Blizzard to decide what is "naughty" or not...very vague rule