Technically, it's not that it can be passed through tools intended for ASCII only. It can be passed through tools which assume an ASCII compatible character set

Technically, it's not that it can be passed through tools intended for ASCII only. It can be passed through tools which assume an ASCII compatible character set (character set for which ASCII is a subset), which are 8 bit clean, and which don't make incorrect assumptions about being able to truncate strings at arbitrary points and be left with two valid strings. Which is actually generally true of any tools which had been internationalized with legacy, pre-Unicode character sets like the ISO 8859 series.