std::io::Read doesn't forces utf8. In fact, it does not imply any encoding - It's just a stream of bytes. It can be a utf8 encoded text file from local disk, euc-kr encoded html from gunzip stream, or even a jpg encoded picture of kitten from the internet.
Read is used for low level abstraction in io context. It only cares bytes, because everything in memory are bytes! Arbitrary typed generic iterator, which should be std::iter::Iterator, can be constructed on top of it.
I think what makes you feeling such is std::io::BufRead::read_line(), which assumes that input stream is utf8-encoded. This is just a simple shortcut for common case, as most streams we handle line-by-line are utf8 encoded. But if it's not your case, you can always bypass such highlevel api and handle bytes directly.