If I offer to paint your house for free, and I do a good job of it but also steal your TV, you actually are entitled to be upset and even call the cops. You can't say, "Hey, I was working for free, and now I want to monetize it with his TV. He's acting so entitled, like I need to ask if it's OK first."
(Obviously I'm not saying they stole anyone's TV, but more extreme examples demonstrate the flaws in an idea better than more subtle ones. The principle of "It's free, so I can do whatever I want and you're being an entitled brat if you object" is simply fallacious.)
Long story short: Working for free does not give you free reign to violate people's trust. Something that isn't OK doesn't suddenly become OK just because you were working for free. If you acted under-handedly, don't be surprised when they call you under-handed on their blog. Being free doesn't immunize you from accusations of under-handedness. It does limit their recourse, but they still have every right to be unhappy.