Maybe they're hoping to make a little money on the pirates...
It's certainly possible—and already done—with iOS.
Likewise, "homebrew" on such a system would probably be like iOS homebrew is currently: an ecosystem of open-source projects where users are expected to download, compile, and sign the binaries themselves using their personal developer certs.
The way this has been impeded on previous consoles is to encrypt retail executables and lock the key in some "secure" coprocessor. These obviously do eventually get cracked, but at that point the system is probably so thoroughly exploited that you can do everything with a retail console anyway.
With this setup, it could easily blacklist known retail binaries and refuse to sign them.
Nintendo accidentally showed a devkit with 64GB of internal storage rather than the 32GB you get on retail units.
1: Nintendo portable systems have sold really well, so the market could be large. DS sold 154 millions units, 3DS has sold 65 million units so far. Of course no guarantee Switch will approach that. It is more risky with the hybrid approach, and continuing stiff competition from smartphones and tablets.
2: Handheld/portable systems are often better fits for indie scale games. Smaller/cheaper/shorter games.
3: Less AAA competition. Nintendo systems haven't been getting the biggest, latest greatest AAA titles from the large publishers, so indie games have more room to stand out.
I believe the Xbox One devkits are just production units with different key material (which prevents commercial titles from running, and allows developer-signed bits to run). Theoretically any Xbox One is a devkit, with the right software provisioning.
Most release games run up to 5mb(or less) to the system memory limits so you can't turn on debugging and waiting for LTCG builds which can take 30mins+.
The problem is likely just that it becomes easier to install third party software if there is some kind of Dev mode inside retail units...but Nintendo's locked down hardware gets jailbroken anyways so I don't see the point in hiding it.