The API though is only public because that is what previous owners allowed. There is nothing illegal about making it private or only allowing certain third party platforms to have access to it. Does it suck they are making it that way? Sure. But don't try to claim anything they are doing is illegal. They are just securing and monetizing the product that they purchased.
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Curseforge has a website dedicate to using their API. Even if it was at one time reverse-engineered they made it publicly accessible with documentation.