Actually, the practice of returning tray is good upbringing, especially if you have your own children tagging along. Whilst I was training in Italy, Japan and working in Taiwan, this was a standard practice and all the locals were doing it. I inherited this practice and continue to do so when back in Singapore and also at all the countries wherever I go thereafter, even until this day.
My wife and I will always use tissues to even wipe our table before we leave the place and if there is no tray return facility, we will always piled up our used utensils and crockery neatly so that it'll be much easier for the cleaner to come and collect them. I can still remember donkey years ago when I was in a Jakarta's KFC. After my meal, I emptied my tray at the usual tray return-cum-disposal counter and everyone sitting near it was starring at me. WTF?