How do you think the legitimate telemarketers get the list? They buy it, that's how. The list is sold to the pests so they know who to not call. All telemarketers are required by law to buy the list and stop calling phones on the list. For a scammer its one more law to break.
I've read that the list is fairly expensive, much more expensive than the robo-dialer programs most scam telemarketers use to call every number.