Have not seen the campaign, but have noticed the tray return points, so have been returning my trays whenever possible. Doesn't take much effort really. The number of cleaners don't seem to have changed.
The 'they might lose their jobs' angle sounds convincing on the surface, but if you think about it, returning the trays is a very small portion of what the cleaners do. Besides, where does one stop? Why not make the table as dirty and messy as possible so that the cleaners have more to do, ensuring their job security, maybe even create more jobs in the process?
I think a scenario where cleaners will lose their jobs is one where patrons have to wipe the table, clean up any food scraps/stains/spills on the table, seat and floor, return the tray, clear the food waste from the utensils and tray, sort the dirty utensils for washing, wipe the tray, bring the dirty utensils to the kitchen for washing...etc. Those of us who do all these in our own homes should feel very guilty then, and stop such behaviour immediately, as we are depriving part/full-time cleaning aunties of a job.