I think I first heard it from Dan Gelbart. The usefulness of a network varies with the square of the number of members. For example, if you were the only person in Vancouver who owned a telephone, the telephone would be of no use at all to you. If there was one other person with a phone, then the telephone would have some small level of usefulness to you. The same rule applies to the web, and the better the nodes of the web are linked up to each other, the more valuable the web becomes.
Links then are in a sense the infrastructure of the web.
From this point of view, it seemed rather parasitic to charge for the links that you create when what gives these links value in the first place is everybody else's free links. This argument was unfortunately lost on Dominion Yellow, despite it's having raged over a week or two on van.general. The second, and more self-serving argument against charging for links is that a database that doesn't charge will be more complete than one that does charge. It can have all those members that the other has, plus those members who do not wish to pay. So any money you squeeze out of the membership side of your database ends up reducing the overall value of the database.