"If you and I are neighbors and we agree that you I will pay you 250$ a month to not park you car in front of my house (while you legally have a right to do so), should that not be allowed?"
I don't know how it works elsewhere, but in the UK there are a number of legal rights that cannot be signed away. For example, tenancy agreements with illegal clauses are simply enforceable for those clauses, whether it's signed or not. Another example; one cannot consent to Grievous Bodily Harm, even if you really want to (this one was taken all the way through the legal system when someone consented to having a nail hammered through a rather sensitive part of his body).
I don't know how it works elsewhere, but in the UK there are a number of legal rights that cannot be signed away. For example, tenancy agreements with illegal clauses are simply enforceable for those clauses, whether it's signed or not. Another example; one cannot consent to Grievous Bodily Harm, even if you really want to (this one was taken all the way through the legal system when someone consented to having a nail hammered through a rather sensitive part of his body).