They are allowed, but interface names are limited to IFNAMSIZ characters, and that's defined to 16 on Linux (and can't realistically be changed), of which one must be the trailing 0. That means you have only 15 chars. A MAC address formatted in hex requires 12 characters. That means you only have 3 characters left for the prefix. An underscore is hence a luxury you can't afford. WE could use it for the other policies, but 15 chars for them is not plentiful either (think about USB device paths!), and uniformity is kinda nice too.