The data isn't secret, but commercial access to the database is restricted. If you want to find a few postcodes (I think <=5 per day) it's free. More, or if you want a manipulatable copy of the dataset you need to pay.
The only thing it does is keep postcode methods of geo-location unavailable to the UK public. Canada, Australia, the US and most of Europe are already moving forward here because they don't have this restriction... the UK loses out.
This isn't capitalism, nor is it socialism. It's a kind of corporatism in which the risk―the money spent speculatively mapping Britain, arguing in Parliament, drawing up postal code boundaries―is entirely assumed by the public, but the reward―access and profit-taking―are entirely given to the private sector.