Yes, because they want you to pay for a monthly or yearly subscription and you just want to read one article and maybe never come back to their site. But if I can do a 1-click "pay 1/2 cent" to read the rest of the article that would be totally fair
First they want you to sign up for an account. Give a lot of personal data. Then set up a payment model, often recurring. Just to read one article? No way.
Micropayments will save some of that hassle but I don't want to have an account with every news site linked to by HN.
Half a cent per article is a plausible target for all the weak/cheap sources who can't possibly afford to put up a paywall right now because nobody would pay. It's not going to be used by the currently paywalled sites, their business model relies on getting a not-micro amount from each customer.