Because your argument only works if contracts are *never* enforceable against third parties.

Suppose I offer to sell you a car for $1,500. Then I say, "I tell you what, you can have it for $1,200, but if you drive it on a Saturday between 2AM and 3AM local time, you must pay me $15." Is that enforceable?

You bought whatever your agreement with the seller says you bought. That agreement includes the laws of your jurisdiction unless you negotiate otherwise.