The key section is 61.159 (b)(4). I think the wording they may be using is "or as second in command performing the duties of pilot in command while under the supervision…". While its dubious the fos are actually getting this kind of practice, it could technically be argued. With the advent of the ATP rule we'll be seeing a lot of this in the coming year. Dovetails nicely with 61.167 (b)(1) stating we may instruct and sign logbooks of other company pilots.