If you clone the environment at startup, then you get a situation where code in the same binary can see different values depending if it uses libc or Rust's std. It's also no longer the same environment as in the process metadata.

Using a copy by default may have worked if it was designed as such before Rust 1.0, but Rust took the decision to expose the real environment and changing this now would be more disruptive than marking mutations as unsafe.