If You and I make an agreement to transfer ownership of a vehicle, the vehicle becomes my property, and is no longer yours. You haven't lost any rights, nor have I gained them. If you, or a third party then tries to drive off with my car, then you or they are violating my right NOT to be stolen from, and most certainly I can enforce that.
Why aren't they violating my right not to be stolen from? Why does your right not to be stolen from extend to cover the car? And the answer is that the right extends to cover the car because of the contract.
Call it extensions, call it whatever you want. But the fact is, contracts create rights, or extend them, or make actions violations of rights that wouldn't otherwise be. Whatever. The terminology is not important. The point is, rights obtains contractually are real rights and in fact most rights are obtained in this way. If your boss doesn't pay you for your labor, he's violating your rights because he agreed to pay you.
When you buy a CD in the United States, you are entering into an implicit contract with the seller of the CD. This contract incorporates the laws of the United States to set the defaults. You are, of course, free to negotiate other terms, but the law will have to set the default in every society. Otherwise, there is no way courts could enforce contracts.