The recycling bins at my apartment in Portland have a sign that says "no pizza boxes" and I finally asked why
Honestly I always tossed my greasy pizza boxes in the recycling anyway because I figured cardboard is cardboard. Last week the building manager put up a new sign with a photo of a cheese stained box and the words "this ruins the whole batch." I asked him about it and he said one oily box can mess up a load at the plant, so now I rip the clean lid off and trash the greasy half. Feels weird but he swore it actually helps. Anyone else's building get picky about this stuff?