When someone releases their software under GPL, they still retain certain rights rather than granting them to the recipient - notably, the right to sell it commercially without providing the source, and without requiring the buyers to adhere to the terms of the GPL. This is clearly a valuable right, seeing how many companies have business models that are built around dual-licensing GPL'd code for commercial proprietary use.

Strictly speaking, the only license that doesn't have the copyright holder retaining any rights is the lack of one (i.e. releasing to public domain). If you're not releasing to PD, that's necessarily because you want to retain some rights. And then it's just a question of which ones. GPL has one answer, something like MIT has another, and Commons Clause has another still. I fail to see what makes some of them proprietary, while others are not.

From my perspective, if I can get the code, hack on it, and release the changed version to others who can also do all of these things in turn, that's enough to make it non-proprietary already. Proprietary is when the software is closed source outright, or the source is provided for "educational use only" (i.e. no derived works allowed), or when derived works cannot be redistributed. Licenses that allow redistribution of patches, but not original code with patches applies, would be the grey territory.

I'm not an expert by any means, but if the Commons Clause's FAQ is accurate, the copyright holder is only retaining a very specific right to commercial use, not all rights to commercial use:

"Commons Clause only forbids you from “selling” the Commons Clause software itself. You may develop on top of Commons Clause licensed software...and you may embed and redistribute Commons Clause software in a larger product, and you may distribute and even “sell” (which includes offering as a commercial SaaS service) your product...You just can’t sell a product that consists in substance of the Commons Clause software and does not add value."

I see how that restricts my rights as a user, but those are rights I simply don't care about. Back in the days of dialup, people were able to make a few bucks by selling Linux CDs to help other users get started. But what examples are there today of unmodified free software being sold by someone other than the primary developer, aside from various app store scams?

Not at all. MS shared source license didn't permit the creation of derived works at all, much less redistribution of them, for any purpose (commercial or otherwise).