"Why in the world would these people want to buy plastic at those prices ($500 to $700/ton) when, as you say, they can simply head to the landfill and get all they want for free??"

Here's a thought...could these people be the ones buying the 8% recycled plastic?? Probably cheaper than buying virgin plastic.

Could this be the recycled plastic that goes into such things as carpets and other products what have a sticker on them that say they reuse a certain amount of recycles plastic??

"Should they have consulted with you first so you could have told them it was free?? "

Ah, no. I am sure that these people are smarter than me when deciding what's best for their business. But I would suggest that they consult with you since according to you, you know it all.