If you're a woman at a bar, and a man offers you a free drink, it's almost never free. There is usually the expectation that you'll get to know each other, flirt, or possibly hook up, and even if that's not tacitly expected free drinks from strangers can be loaded with silent assumptions.
She opened her post by stating that many men who buy women drinks are hoping to purchase "a lowering of woman's defenses" and that has only been confirmed by her personal experiences.