Agreed. The point of having a spec is to ensure that users get the same results regardless of tool. So I’d say everyone should follow the spec.
What’s not clear to me is that the current wording of the spec is actually the right solution. It looks like it’s never been proven in a real-world tool, which suggests to me that implementing the spec as is could be a worse solution than changing the spec (either to reflect pip’s behaviour, or to specify something better than what pip does).
And just to further complicate the matter, we’re very close to the question of whether standards should be allowed to dictate tool UI. The various SHOULD statements in the spec are basically tool UI recommendations, and I think they should probably mostly be omitted, or put into some sort of non-normative “UI discussion” section. Right now they mostly read like “we aren’t sure the mandated behaviour is right, so we think tools should offer a bunch of options to work around that”.