When I create a Joomla Core Custom Field and select the option "only in subforms", Joomla hides the option "Categories", because it makes no sense to assign a subform field to a category. RL Articles Field shows this option.
For me the category field (on the right) acts in exactly the same way for Articles Field.
Those fields are standard and shown for all fields. There is nothing in Articles Field that would change that behaviour.