Perhaps if you put all the mucky landfill stuff in with the recyclables, they end up dirty. And once it's been in your bin for a week or two, then taken to the recycling centre, it's caked on and cannot be cleaned. They ask us to clean our recyclables anyway. It's easy to rinse a yogurt pot under the kitchen tap, but once the yogurt has dried on, it would need scrubbing, or some kind of powerful hot water jet like a dishwasher, which would just melt the pot.