For instance, even if I could buy a TV and then return it for free, why not return it and charge a fee? Most people are willing to pay the fee, even if they wouldn't return it, so what's the incentive for any vendor to not charge the fee?
Your suggestion proves my point! With social contracts I don't need to pay this fee, but by getting rid of them I do; I'd be getting charged for other people's anti-social behavior!
Stepping back, social contracts are forms of trust at the societal level. Any economist or business person will tell you that being able trust others dramatically lowers transactional and operational costs. I want to be trusted so that I can take advantage of those lower costs.
ADDED: Furthermore, research has has shown (e.g late pickups at daycare) that when you add a fee to an anti-social behavior it can actually make the behavior more likely because it's now a transactional activity.
I've often wondered how well it would work if you went "right well, if you pick them up 15 minutes late today, you've got to pick them up 15 minutes early tomorrow", or "... drop them off 15 minutes late tomorrow".