Dioxins are generally the biggest emission concerns and they are prevented by higher temperature burns. Plastic is also likely higher in energy density which could potentially both make it more economical than a given baseline unsorted garbage depending and serve to keep the temperatures high. Although ironically not being /very hot/ when burning organic matter risks dioxin formation between about 200 and 600C.

Probably they the high quality is mixed with low and the incinerator operator just buys the contaminated lot of it.

Cleaner burn?

They want only the plastics, and not the water, rotting food, literal shit, construction waste, etc. that ends up mixed into the cheaper waste streams.

Those things don't burn as well.