Part of the moderator's job is to ensure that suitable avatars are bequeathed to the posters so we have personality in the forum. They are gifts from the amods to the users. My avatar is included in that and was a gift from Noodle.
However, when the user does not want the avatar, it is their prerogative to reject it. But without an avatar your posts are naked and bland. This is also your prerogative, but wouldn't it be better to be clothed? I recommend picking a suitable avatar, but you shouldn't have one forced upon you against your will.