In a free market, price is dependent on supply and demand. The equilibrium is determined by the supply, which trends up based on quantity that can be produced for however much work; and demand, which trends down based on the amount that is desired, and the price one is willing to pay for it.

Now apply this to something abstract like "information." It costs nothing to produce a thought. And in this day with the internet, an idea can be distributed to anyone who wants to read into it for the minuscule cost of bandwidth, and a little bit of time one may set aside for the sake of learning.

So imagine a supply curve trending up at an unreasonably slow pace. Almost just a horizantal line representing a massive Quantity that can be reproduced, with a price which will always be next to nothing. The demand curve will intersect the asymptotic supply curve at a price one could just call "Free."

So, just economically speaking, "Information is free" makes perfect sense. The minimal opportunity cost of copying data, is something that truly would be Free if it weren't run by profiteering gluttons, who are artificially jacking up the price of information sharing. Another shift that has occurred with the internet, however, is the information sector being in the hands of the people now. With hacker groups such as Anonymous, protesting to maintain freedom on the internet, it's probably going to stay that way.