Megan goes over her reasoning to legalize drugs, and it's all based on her experience with her friends. Perhaps her friends are not a representative sample of the US? If you are going to talk about policy, you need more than anecdotes.
Eric, go back and watch any of Megan's old diavlogs. I have no idea if this is the way she is in her writing as well, since I am simply not a strong enough person to put up with that sort of self-inflicted torture, but when she speaks here, the vast majority of her opinions appear to be based entirely on anecdotes from her own life, usually involving her mother, high-quality food markets, and her 20's-to-30's group of blogger friends in New York and then D.C.
The absolute worst was her first vlog with Yglesias - it turned completely into a autobiographical exercise in narcissism of the highest caliber, as their stories about how amazing D.C. and New York are simply spiraled into a positive feedback loop of interactive cowanking. Ugh.
Mostly, I think I listen to McArdle to make myself feel better about the vast majority of ways in which I disagree with her, since the absurdly flimsy structure of her belief structure makes me feel that I must be doing something right.