My intention when I posted was to share the tri guy's hilarious reply and attitude, not to start a discussion about the pros and cons of drafting strangers. That said, opinions are welcome. Mine is drafting is part of the sport of road bike riding and not so dangerous (at those modest speeds at least) as some think. I met some great cyclists that way. About strangers, if they want to get on my wheel, no problem, if something bad happens probably will be to them. As for me, before drafting a stranger I assess quickly how they ride, if they keep a straight line, don't brake or surge, etc.
Anyway, the end of my story was that after 3 or 4km they upped the pace to 40-42kph, the MTB guy holding on for dear life now, so after a bit I thanked them and watched them vanish in the distance...