Here is the legal problem. Unless it is posted, it is free. A contract, to be considered legit must be disclosed and agreed to by both parties. Had it been posted, you would have had the option to choose to not eat there. Since it was not posted, you never agreed to the charge, and therefore do not have to pay. I would not have paid, and they would have had to come after me, which they would loose in court based on well established contract law.
Per the post above, most assume the cost of infrastructure is paid for by the cost of goods, SAME as it does on land. In places where the infrastructure cost is high, such as in the city, there are parking garages with posted fees. Even so, so businesses understand the need for free parking, and pay for expensive land, in order to be able to do business in that area. (such as the McDonald's in Downtown Chicago)