I've seen signage to that effect at buffets before. At least one of the Chinese buffets in Houston (of which there are about a billion) has such a sign, though I don't remember which one. I don't know that it's enforced...just a not-so-subtle way to say, "Don't waste food!"

These are how buffets are where I grew up. When I moved to the US, I was surprised that buffets here allowed me to leave food on the plate while I went to get another plate of food.

Often happens at all-you-can-eat sushi places; personally I know of three or four in Montreal/Toronto where this is done.

They're the kind of places where you order small dishes off the menu, and you can just keep ordering them until you're done. No limit on the number you can order simultaneously, but if you call for the bill when there are uneaten plates they'll charge you some nominal per-plate fee for each one (this isn't for leaving one or two pieces behind - it's for when you order a plate of ten pieces of sashimi and eat two).