I never understood the reasoning behind this, though.
At my day job I could spend five or six hours with a new f/o (170 hrs total and 1 hour on type for base training, plus the sim course) briefing them on everything they needed for line flying (flight plans/OFP/FTLS etc, plus an aircraft visit to go through the emergency equipment and teach how to do the PDI. Next day we would fly four sectors, teaching how to operate in controlled airspace, London TMA, practising and teaching all the things an airline pilot needed to do, but none of that could be counted as instruction to keep an instructor rating current. Yet on my day off I could spend an hour in the right seat of a Cessna 172 with a private owner who had 1000 hrs on type and needed no real instructional input from me during the hour required with an FI for his SEP currency. Yet that hour, where I used no instructional skills at all DID count towards keeping my instructor rating current.... bizarre.