I've found that this is generally less true than I imagine it to be. A lot of law is not actually so difficult to understand. Legalese is still English, and Google is great for figuring out the odd meanings of certain terms of art.
Most of the exceptions I've come across (e.g., buying a house) are IMO actually intrinsically complex transactions. I could undestand, but it'd take so long. And the time taken would happen even if we only had the laws we needed and all those laws were written in plain english. In software terms, lots of essential complexity.
Going back to the article, the problem with Facebook's terms and conditions is definitely not understandability -- see https://www.facebook.com/policy.php. The average high schooler could understand. The problem is people caring enough to 1) find, 2) read, and 3) inconviencance themselves if they don't like the policies.