I seemed to remember reading something similar about Austin's buses when they went fare-free in the 80s. I found this linked in the Austin CapMetro wiki page: http://www.nctr.usf.edu/pdf/473-132.pdf
This reminds me of MetaFilter, where a one-time $5 payment is required to post. Just this small speedbump seems to do a pretty good job of keeping opportunistic trolls and vandals off the site. "Free" is too often an invitation to abuse.