I think the has been some info lacking when this decision was made.
In places where tray return is practiced, everything served was either eaten or disposed, except the trays of course. There are no ceramic bowls, plates or metal utensils. Just paper cups, plates, cups and plastic utensils, with the occasional disposable chopsticks. Hence users simply finished their meal, dumped everything on the trays, wipe up whatever they've spilled on the tables and put the tissues into the trays as well, and carry the wastes to the bin, dump them, and stack the trays atop the return station.
And this should be practiced because we are people living in a modern urban setting, not whether one is a Singaporean or foreigner. And adults should set the example for children, not left to the GAHMEN for direction!
Cheers!