Having worked in the airline industry, I can tell you, this is not a case of airlines making a poor assumption. Airline systems are generally incapable of dealing with ASCII, let alone Unicode text. They typically use EBCDIC. That's why you never see lower case characters in your name on a boarding pass.

That makes a difference when you get to the airport and they won't let you on the plane because your ticket and your id don't match.

Does this actually happen? Because given the fact that airlines have been incapable of printing names that don't can't be described by EBCDIC since, well, forever, I'd assume that they're willing to ignore cases where the name on your ID contains characters that their computer system literally cannot represent.