Perhaps, but in this particular case - that's probably a perfectly fine trade-off. The "worst case" exploit of this scheme seems to be that someone other than a subscriber gets to read the Magazine. The marginal costs to Marco for that occasionally happening are practically zero - someone who is prepared to steal passwords or read other peoples email accounts to get access to a website was super-unlikely to ever become a paying subscriber anyway. Marco has made it easier for the people who are/might be paying subscribers, and the tiny cost to him of potentially serving up his content to people who steal credentials from real paying subscribers. Since that cost is so low, and the added convenience is of value - why would he _not_ choose this?
(Hell, even if the only result of this scheme was this discussion on HN, I bet it'll turn out revenue positive in spite of the increased fraud risks...)