This is much more excusable for email providers to prevent phishing. There are a ton of unicode points that indistinguishable from ascii letters. There are other security issues that can arise as well. Here is an example from spotify https://engineering.atspotify.com/2013/06/18/creative-userna...
I should have specified - this was (is?) for passwords, not usernames. I'm much more sympathetic to limited character sets in usernames, but I don't see much valid reason for doing so with passwords