Good observation! I had been the one who screamed to SQ since three years ago about this. It's in the SQ formal complaint letter.

It seems that the FAs had been trained to collect menu whatever left by the pax. But, the practice is that the FAs will collect them long before landing, i.e. when taking the meals order before landing. I have a habit since then to tuck away the menu, somewhere not in the seat pocket.
I did post my rant about an In Flight Supervisor who demanded my SQ1 menu before landing in HKG. I sent e-mail once I was in the lounge at HKG, and SQ YVR confirmed that the reason given by the IFS (yes, the HKG-SIN flight is full in F) was a BS. So, I am sure now in my profile there is a note saying don't make this pax returning his menus.