Having worked in both, I'd say they both chose ideomatic solutions:

Rust: I can't prove this is utf-8, so if you want to use it as utf-8 you'll need to tell me what to do if it isn't.

Python: if you're in the common situation where everything is utf-8 and you want to just work with strings, go do the simple thing. Or you can be explicit about wanting to work with bytes, and that's good too.

(Though I think Python should throw an error instead of silently omitting non-utf8 files.)