This style of DoS harms more than the target (Score 5, Insightful)

Most Denial of Service attacks affect more than the target itself. If I'm attacking example.com, then all machine between me and that machine are busy handling my traffic. An intentional DoS'ing may not be much worse than a slashdotting for an ISP, and is usually easier for them to shut down. That costs them money, but it doesn't take too long, and the only real cost is downtime of their other subscribers, which since most sites are independent of other customers or have so little bandwidth compared to the pipes coming into the ISP, doesn't affect other customers much.

In the case of signing up a spammer or other unscrupulous individiual to catalogs and other physical mail, the companies that are sending these items are directly bearing the cost of your DoS. Sure, Sears can probably afford to send out one more letter, but catalogs are more expensive to print and mail. All these companies are getting screwed out of real money, not some potentially (and oft inflated) accounting of how much time/cost an ISP has for DoS countermeasures.

Sure, I think it's great to spam the spammers, but in doing so you harm legitimate companies more than in the Internet world.