The New York Post says Cameron was playing middleman to half a pound of crystal meth—close to $20K worth—that was being moved from California to NYC to sell. The cops caught the drugs before they got here. Then they went to pick up 30 year-old Douglas:
He had been staying at the hotel for some time in a room rented by his father, and when authorities barged in, they found the place a mess and Cameron Douglas "very strung out," according to the sources.
Cameron's had drug problems in the past and Michael Douglas went through rehab himself in the 90s, because addiction really does run in the family.