> Actually, they are explicitly specified as ASCII
They are not, only new headers added after RFC 7320 are ASCII, pre-existing ones may contain high-bit characters which implementations compliant with 7230 version of HTTP/1.1 SHOULD not parse. They stopped short from making it a MUST so that applications compliant with RFC 2616 (where they were to be treated as ISO-8859-1!) was allowed. Or, more simply, they stopped themselves from making RFC 7230 describe new, incompatible HTTP/1.2.