There is a considerable difference between getting requests and providing support. You don't have to provide support, but it's a little hard not to get requests.
Well, you can disable the issue tracker for a project on GitHub. You're not even required to publish a project on GitHub, you can host it somewhere that doesn't offer any way to contact you.
I don't think GP was saying that anyone is obligated to provide support, just that receiving support requests is intrinsically part of publishing software as source. The only way to avoid receiving requests is to abstain.
To be fair, the reason the curl author is being contacted is because (IIRC) he advertises his name and ways to contact him in curl. The point of advertising yourself is to get people to contact you, and you can't control who does (but can ignore them).
