So seatbelt fastening IS required at take-off and landing.

Maybe next time I will say something, but I don't like to make waves, and I don't want another passenger to be angry at me in the confined space of a plane for the duration of a flight.

In one case, the guy was Asian and may not have spoken English (international flight). The FA told them twice to fasten it (on the walk down the aisle and then on the way back up the aisle). But surely they should have some recourse for language barrier. Anyway I think he was just pretending and blowing her off and she didn't push it.

In the other two cases, the passenger "tricked" the FA into thinking it was fastened (although I can't believe the FAs did not know). They were very obese passengers that could not fasten the belt. One held the ends of the belt, one in each hand, and laid a pillow over his hands to obscure the unfastened ends. The other one lifted up his abdomen and "tucked" each half under a fat roll, then let the roll back down, holding the ends in place so all you could see were the two sides disappearing into his body.