Presumably uploading a new avatar is something one might make use of with the users:update API scope. (Which isn’t currently possible?)
With more varied and potential future avenues where uploads might be used, it probably does make sense to create a separate scope and put the onus on the user to choose the appropriate scoping. It’s clear for users when an API key will/won’t be able to upload and avoids potentially missing situations where uploads could be used if/when extending the APIs.