They make money from their cloud services: storage, GPUs, dataset hosting, model training, model running, code sharing, model distribution. They're a one-stop shop for nearly any ML cloud solutions you'd need.
The transformers library and other products boost their image, making more people know and trust them, making more people pay to use their cloud services.
That's not to say they won't screw us if we start trusting them too much and relying on them with no alternatives. Or that they'll ruin a good thing in order to make even more money.
But I don't really see how they can screw us right now. All the code they release is open-source (transformers under Apache 2.0), and they data they provide isn't theirs. They're the de-facto standard in every academic and open-source setting I'm familiar with; it doesn't mean these people need to use huggingface, these people want to use them. And, they have a way to make money (cloud services) which not only should be profitable on its own, but encourages them to be charitable and keep good reputation to get and retain customers.