If I understand the history correctly, the CAA views itself as the generator of the content of these documents which are rules or guidance or other useful stuff. In the olden days the only way to get this information out to the masses was to print it. The price of the documents was only to cover the costs of printing and distribution (and profit for the company that did it) - it's just that the companies that did the printing and distribution were not very efficient. When the internet became available this offered another way to distribute the information at much lower cost - both to the CAA and to 'customers'/users. There are still companies that will sell printed copies of the documents and the price should reflect little more than the cost of production if that's what you want - the problem is that fewer people buy commercially printed versions and so the unit costs go up.
There is another advantage to having the documents available on-line in that you can be sure it is the up to date, definitive version rather than a copy of last year's issue of CAP 413, or whatever, that the flying club or shop is trying to clear before they buy any copies of the latest edition.
As for the AIS stuff that TheOddOne mentions, the reason that NATS operates large parts of the AIS is because it is an obligation in the licence granted to NATS by the Government when the company was split from the CAA. It's not uncommon for AIS to be provided under contract although he's quite right that the AIS website is not the most user-friendly or functional one around!