As has already been explained, that is only true because of circular reasoning.
D&D assassins must be evil only because the rules say D&D assassins must be evil. There is nothing about the abilities granted by the class which makes the class inherently evil.
It's an arbitrary rule. And it's a bad rule, because it undermines the game's ability to represent the kinds of characters typical to fantasy adventure. The non-evil assassin protagonist is a stock character of those kinds of books and movies- it should be supported by the rules.