Military personnel agree to subordinate their constitutional rights to UCMJ when they swear their oath. They can't even go to a political rally in civilian cloths.
Reserve and Guard personnel can do so, but not in a forum where their military duties are identified in a public forum. For example, as a Naval Reserve officer, I was free to participate in National political campaigns, but if they referred to my active duty Reserve service (i.e. Commander SFS, USNR states "..President Bush is an illegal ursurper of his office ..", or "..President Clinton is a crook..", I'd be in hot water. Even if I was retired, I'm pretty certain the UCMJ would prohibit a retired military of member from, for example, insulting the President, etc..