I do think that if you distribute a binary along with the GPL license text, then you've obligated yourself to provide source. If you distribute the binary of your own program without the GPL license text, then I don't think you're obligated. But I'm nowhere near being a lawyer :)

Thankfully in practice none of this really matters.

The person licensing the code as GPL generally wants to share it. The license becomes much stronger as more people contribute code under the same license (but their own copyright), since it means everybody is obligated to share under the GPL (or else strip out all code which they don't own copyright over). And as more people use/contribute, the value of the code generally increases, so it's better that the protection becomes stronger.

And of course, supposing there's a GitHub page for a project with a GPP license attached, then anybody who has cloned that repo is automatically granted all those rights. The owner of the copyright (assuming they own 100%) can still release the code under a different license or terms, but they can't revoke the rights granted to anybody who already received the code.

All that said, I think if I incorporated an AGPL piece of code not owned by me into my homework assignment and then let you access my homework assignment over a network, that would probably obligate me to give you the source code if you asked.

Yeah, probably true!