Because the language doesn't define what happens if you write to memory behind the runtime's back. It _does_ define, in detail, what occurs on panic. It's a legal thing to do. That some consider panicing in bad taste or whatever does not change the fact that panic is a supported feature of the language.
Re:Redox OS Crash Challenge
by steveklabnik on Tuesday January 23, 2018 @02:19PM
(#16216568)
It also defines that it may abort, and so culturally, people use them for only what’s intended, as otherwise, it limits your audience.
If hypothetical unreasonable libraries are in scope, they can just open /proc/self/mem for writing (in safe Rust!) and violate memory safety, and that's not an argument that Rust isn't memory safe. So why should hypothetical unreasonable libraries be an argument that Rust has exceptions?