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Not just sorting, but contamination.

Cardboard is recyclable.

A pizza box soaked with grease ins't, and can contaminate other potential recyclables as well.

Isn't it? All reused cardboard gets soaked and cleaned first, I'm sure the recyclers can deal with oils and other such contaminants.

Where I live (Germany), the recycling bins for paper and cardboard explicitly forbid fat-stained paper. (Not that anyone cares, though.)

A single pizza box can ruin/contaminate an entire bale of cardboard.