I think you’re a little incorrect in assuming the MacOS “welcomes” sideloading. By default; there’s all sorts of restrictions applied to less official or alternatively sourced apps and data.
For example; blocked by default is any untrusted developer or invalid certificate. And then even if you opt to allow (and/or weaken security); there‘s still more to be done to launch these (right-click open, permissions, verification) because they haven‘t been granted the same level of trust.
And if you use windows; you’ll notice something similar, and for documents too. Perhaps not as well implemented, but which is a consequence of constantly increasing security issues from previously not limiting (critical) access.
Then there’s the “can buy a brand at multiple stores“ analogy, which if you think about it, is actually the opposite of the App Store you’re trying to relate it to:
Yes; you can buy a Samsung TV in Best Buy. Can you buy a Best Buy own brand product in a Samsung store? Or will LG sell you a Samsung, and Samsung sell you an LG?
Or to put it the other way round; you can get Samsung and LG apps that run on your iPhone from the App Store. You can also get apps from LG and Samsung that run on Android from the Play Store.
Presence on one store doesn’t preclude presence on the other.
The point is macOS isn’t locked down. Sure there are protections in place as you mention, which are welcome, but at the end of the day, the user is given the choice and ability to sideload… not only apps but also kexts and now dexts. ipadOS 16 will now allow users to load dexts, which is good for audio interfaces and other thunderbolt hardware. So apple is slowly opening up.
The need for security on the Mac is just the same as on an iPhone or iPad, as a mac has access to your iMessages, photos, iCloud data, wallet data, Microsoft office data, and so on. Again the m1 chip in an iPad is locked down but the same exact chip in a Mac is not. Why ?
My only point about LG Samsung tvs was just that a prospective purchaser of those devices aren’t restricted in where they can purchase those legal devices. They can go to Best Buy Walmart Amazon etc.
But the same is not true for all apps. For example, take wifi explorer. That app exists on android and also the macOS App Store. Allows you to see your wifi radio environment for better channel planning. But it is not available on the iOS App Store. For whatever reason apple has restricted and prohibited apps like it from being offered on the App Store. A perfectly legal and helpful App that is on the macOS App Store is prohibited from running on an iPhone. Why?
Governments have to weigh whether the restriction of trade (e.g., between me and the developer of wifi explorer) is legal. Or whether apple and Google should be the sole in-App Purchase fee collector and operator. Especially when those same App Store operators and in-app purchase collectors, as in the case of apple, compete with other apps in the App Store. Like Apple Music vs Spotify. Or Apple TV vs Netflix.
I agree that apple should be paid a fee and earn a profit from operating the App Store. But apple doesn’t charge an in-app purchase fee for physical goods like Uber or Lyft. Why? Also, if Netflix or Spotify are savvy enough to have their own in-app purchases backend, why are they restricted from being able to use that so as to lower costs to consumers?