You mention that "Note that those surprised programmers are actually Rust compiler authors" but I can't figure out which of the many links is to some "surprised programmers" who are actually rustc authors, and so I don't even know if you're right.

Rust's safe subset doesn't have any UB, but the unsafe Rust can of course cause UB very easily, because the rules in Rust are extremely strict and only the safe Rust gets to have the compiler ensure it doesn't break the rules. So it seems weird for people who work on the compiler guts to be "surprised".