I despair. What a load of ****e.....'probably be more tired than if you'd watched a Grand Prix'! Along with most other people I generally move around a little on my days off, I don't regard myself as tied to the settee in case I get a bit tired. I might do some work in the garden, work on the car, even go for a walk - all more tiring than instructing (have you ever instructed?).
If you want to deal with fatigue you have to get a lot more serious than just picking on people who might like to fly on their days off, you have to take on the Michael O'Learys of this world who have turned passenger flying into the cattle market it is today, by squeezing every last drop out of all the component parts, incuding pilots. If you're going to use resources to combat pilot fatigue then instructing on days off is a big fat red herring, you need to grasp a much bigger nettle than that. And who will do that.........?
Pull what, the day somebody tells me to do nothing but rest on my days off is the day I kick them in the bollocks (purely for relaxation of course). The CAA taking the view that instructing is more or less recreation for airline pilots is a very pragmatic approach and saves them wasting time and money barking up empty trees.