If there is a legal interpretation to the contrary, I'd certainly like to see it. However, the regulation provides NO relief for the requirement that a flight instructor sign the logbook for all instruction given.
That the instructional flight is required by the FAA, insurance, friends, family, romans, countrymen, the student him or herself, an act of congress, the whim of a passing gentleman, or the ides of march, is irrelevant. The fact that the instruction takes place is quite relevant.
The student is not required by regulation to log any time not used toward recency of experience or a certificate or rating...but the instructor most certainly is. The instructor must sign the logbook of each person to whom he provides instruction, period.
The instructor must also maintain a record of that instruction for three years. You'll note that the regulation makes no relief statement stipulating that this only applies to instruction given for a certificate or rating, or for the purposes of recency of experience. Not at al. The regulation provides the requirement in the imperative; the instructor MUST sign the logbook of each person to whom he provides flight or ground instruction. Even flight never takes place and it's only a ground session, the instructor must sign the student's logbook. This is not optional.
You may find a location where a "checkout" may be handled by a private pilot or the holder of a commercial certificate, but doubtful. The company is going to require that the checkout be conducted by the holder of a flight instructor certificate. A flight instructor during any given instructional flight may be talking, observing, evaluating, demonstrating, explaining, directing, aiding, assisting, etc...but is still acting in the capacity of a flight instructor. In this case, the purpose of the flight is for a flight instructor to provide a checkout, and additional instruction for the purpose of obaining local flying privileges in a new aircraft (eg, Cessna 150 to Cessna 172, etc).
The flight instructor MUST sign the student's logbook. TSA has taken a renewed interest in any flight instruction given, and records should be detailed and exact.
If you believe that no records should be kept of any instruction that's not specific to a certificate or rating, then perhaps those wishing to only receive instruction in how to take off, but not land, should go un noticed, have no records kept, and be exempt from any of the new screening requirements? After all, that's not toward a certificate or rating, either.
The instructor must sign the logbook of each person to whom he provides instruction, regardless of the intended use or benifit of that instruction.
Weather the instruction is used toward a certificate or rating is entirely irrelevant to the issue of weather the signature is required. The student is not required to log it, but the instructor is required to sign the student's logbook all the same, and maintain exacting records for himself or herself for a period of three years.
As an aside, a simple checkout I did as a courtesy many years ago for a small flying club turned into a legal nightmare...several years later. The individual to whom I provided the checkout destroyed an airplane of the same type in which the checkout had been conducted. He attempted to land at a remote, unattended narrow airstrip in winds beyond his capability in a rental, and lost control. He had sought no additional training, and my signature was the last in his logbook.
My very detailed records of the event prevented me from becoming entangled in what followed, as I was able to clearly demonstrate my involvement and the extent of it. The signature is required to be in the book. One might argue that had I not signed his book, there would have existed no issue for me...but that's quite irrelevant, as I was required to sign it then, just as I would be now. What protected me were the detailed and exacting records I kept then, and keep now. Sign the book, and document everything.
