If I agreed to such a ludicrous contract, I would be bound by it.
Exactly. In a Libertarian society, IP 'laws' would be implemented as contracts and they would be enforceable.
You bought whatever your agreement with the seller says you bought. That agreement includes the laws of your jurisdiction unless you negotiate otherwise.
We're not talking about laws. We're talking about rights.
When laws set the parameters for contracts, they define rights. For example, say I agree to mow your lawn 8 times over the next 2 months for $900. Then say I don't mow your lawn. You certainly have the right to damages. But do you have the right to compel me to mow your lawn? Unless the contract specifies otherwise, you have that right if the law says so and you don't if it doesn't. One of the things laws have to do is set the default terms -- otherwise you couldn't buy a candy bar without signing a multi-page agreement and courts would have no rational basis to decide how to address contractual disputes when terms weren't in a contract.
Unless agreed otherwise, a contract transfers those rights the laws says it does, as this is what both parties expect. When you buy a CD in this country, you understand that you are agreeing not to copy it because the law does not give you that right. If you wanted to buy that right, the other party would have charged you more.
In a purely Libertarian society, there is no question that IP could be implemented by contracts. The only real question is whether those contracts should be enforceable against third parties who knowingly interfere with them (as in my shrimp example).
And note that absent IP laws, things might never go into the public domain and you might never have fair use rights. In a pure contract IP system, the contract says whatever the author/composer/artist wants it to if they can get the buyer to agree. This is why companies like Microsoft use copyright and patent when they have to, but they much prefer contracts (EULAs) -- because they can choose the terms on those.