[danans]

A lot of cardboard has no recycling value because it is contaminated with other materials.

For example, used pizza boxes soaked with grease are not recyclable. If you're lucky and your city has a municipal composting program, they can be composted, or some places incinerate stuff like this for energy, but otherwise they end up in the trash, or worse, discarded in the forest.

That doesn't stop people from throwing them in the recycling bin though.