Yes, the problem is that you don't really know what you are giving in exchange for "free use". Dr. Wright was also saying in one of the videos that in order to complete a transactions both party need to know what they are exchanging for. He also went on to explain if it is not clear, then the transaction/exchange is in fact "invalid." from legal standpoint.
Anyway, one of my favorite teacher, Prof. Dan Ariely (Behavioral Economics) explains about the special power of "free".
1. Price of Free