Lots of people apparently don't read the instructions for what they're doing. That doesn't mean they aren't responsible for what they do, or for breaking those rules they didn't read. So "nobody knew about it doesn't fly", especially when those rules are about two lines above the dialog box for entering the new name.
Having said that, the rule does go against established rules for realm names, and it is being reviewed, quite possibly to be changed.
However, please remember that the current dev team didn't make this rule. And rules don't ever get reviewed until they come to someone's attention when they get broken. And then only when someone makes a fuss about it and brings it to our attention. The titans are responsible for enforcing the rules AS WRITTEN, and not for rewriting the rules to suit their whim.
