actually, they do have a right to specify the terms they are willing to offer their service for, just as we have the right to agree to them or disagree. And yes, being a free service means they can change them and post them as they desire as there is no legal contract between a free service user and a paid service user. In order for a contract to be valid, both parties must receive "consideration".
Usually that is the form of compensation with money, but it doesn't have to be. I suspect a wily lawyer could have made the case that gaining rights to use your photos for whatever purposes counts as "consideration" and thus changing the TOS should be considered a breach of contract, or at minimum a termination of it which would include termination of any consideration.