You are intentionally muddying the water by bringing code signing (a technicality) and devices for which software distribution was never part of the economics. Nevertheless, yes, I think making ebook readers tied to a single store illegal would be a net positive from consumers. It would let stores compete on their merits as stores and readers as readers. More competition is always good.
Pointedly, my argument is simple: the modern concept of plateform, as has been pushed by management consulting firms for the past two decades and successfully been exploited by hightech companies, is a blatant attack against anti competition laws and I believe the whole thing only started because the US government decided it would only meekly enforce them. Rebranding barriers to entry moats shouldn't make artificially building them more acceptable.
As you rightfully pointed, plateforms seem to be popping everywhere nowadays from game consoles to cars. I don't find that surprising: reducing competition is like crack cocaine to companies. It both means less need to innovate and differentiate and a larger share of customers surplus (theorically all customer surplus but it might be too blatant then). Customers are the ones losing there.
As the whole thing strongly goes against one of the core tenant of modern capitalism, it seems obvious to me it is going to end with more regulations as soon as the political zeitgeist regarding economy will lean less towards extrem laissez-faire. Unless of course, we decide to go full-bore towards nineteenth century style capitalism...