Well, the addon is weakauras - which is free - and the development Liquid does is a derivative product of that addon.
It’s a little more legally complicated than “The WOW TOS says free” - and I don’t think either side is really motivated to fight over this issue - especially since Blizzard presumably would either not be seeking any money - (or they would have to show damages (and the race to world first is actually a positive for them) )