From a purely technical perspective, I believe comparing this to an open Wifi hotspot it a little misleading. My understanding is that the Sidewalk network uses a combination of LoRa and Bluetooth Low Energy, not Wifi, so this isn't something you can just connect to with a standard Wifi device and start using someone else's connection.

Most phones do support BLE so it might be theoretically possible to connect a phone to the Sidewalk network, but I don't think a BLE access point is a common thing, so I suspect it wouldn't be totally trivial to connect via a phone. BLE is also quite short range, and LoRa, which is longer range, isn't commonly available in consumer phones/laptops, although you can easily add LoRa support to a Raspberry Pi or Arduino, etc.

I've also not seen it confirmed, but I'm guessing that Amazon themselves are building protections into the network, simply to stop other devices being able to use it, probably including limiting the endpoints that you're able to reach. It's obviously not in their interests if a competing product line can use the Sidewalk network free of charge, so I'm assuming that it isn't simply providing an open link to any Internet endpoint.

None of this removes the privacy concerns, of course, or mitigates the all-too-typical automatic opt-in behaviour, but it's probably worth being aware of the distinctions.

I tend to find myself slightly torn about developments like this. From a techie point of view, I can very much appreciate it: you have a massive network of devices, probably well over half the streets in North America have at least one Amazon device on them, and they all have LoRa chips in which provide a low-bandwidth network that can reach a couple of kilometres from each device. Of course you want to enable that network, it's just plain cool to be able to allow other bits of your tech to communicate back to base for free on a tiny power budget in almost any populated area. But on the other hand these companies behave so untrustworthily and unethically that it's just plain hard to feel much enthusiasm for the idea on a personal level.