It's partially self documenting. As mentioned elsewhere, there's a vague implication that putting something else there could mean "don't use utf8", where if you put the utf8 X character that corresponds to the check you are still asking for utf8.
edited to add:
This seems related to a problem various "try to sound like English" programming languages (e.g. Inform) have, where it is easy to assume invalid syntax will be valid because it's valid English.
An X is also a commonly accepted way to select a checkbox though. The only sure way to indicate a checkbox as not selected is to leave it empty. That's the true opposite, and actually does work.
