Depends really. I suspect those who've perhaps flown in a premium cabin just once or twice, or perhaps been upgraded, tend to leave them on intentionally as they like them (I know I did the first time I flew CW). Those who travel regularly in those cabins probably just forget to take them off.

The barcoded tags are a b.tch to get off if they've been left on for a long time. I've broken numerous claws (ie. nails!) trying to get them off at check in. Most of us now ask the passenger to remove them to avoid this. (BA don't pay us for manicures )