I see the problem as an issue of trust with people identifying themselves as a potential customer or non-customer (some-one who would never pay for your content).
Because there is no immediately lost revenue when a non-customer views premium digital content without paying (due to the free duplication) the only reason I can see for denying non-customers your content for free is because there are indirect consequences to their revenue. Such indirect consequences might be the devaluation of content due to its perceived lack of prestige and exclusivity. But the main focuses of piracy, music and films do not gain value due to exclusivity or prestige (I struggle to think of anything digital that would benefit from this). In fact one could argue that the spread of these goods creates advertisement by word of mouth and increases the potential set of customers (and non-customers).
Determining who is a valid non-customer is however nigh impossible. The only scenario I can envisage of a provable non-customer is when some-one clearly does not have the finances needed to be a customer. However this scenario falls apart because you cannot prove they would never acquire the finances and become a customer in the future. You can however obviously prove yourself a potential customer by paying for the product.
Ultimately I like to think most people would be honest enough to say whether they're a potential customer or not (and O'Reilly clearly agrees) and in an ideal world I think every-one should have access to anything (spend all your money and you're then a non-customer for other products). However it's the prerogative of any company to only sell their content to proven customers and not give it away to possible non-customers.
There's an interesting story here in HN that debated this conviction: http://news.ycombinator.com/item?id=2490457