If Tasini was actually serious, he'd be suing other places as well.

Huffington Post's Unpaid Bloggers Are Suing Arianna

I mean, I'm not a fan of the HuffPo. I think people like Amanda Terkel and Ryan Grim do some really great work.
Nobody forces the unpaid bloggers to post at HuffPo. I won't generalize anyone's motives. But if Tasini were honestly serious about suing the people who use his freely posted work for their own gains, he'd be suing Daily Kos as well. Too bad that doesn't suit his narrative of choice.