The debate, at this point, is not about intellectual property. The debate centers on what rights you have when you purchase a DVD. Are you buying a license or are you buying a material item? If you are only buying a license, you are correct. If on the other hand, you are buying a material item, in this case a single physical version of the item in question, then you are wrong.
If in fact you are buying a license, when is this license agreed to? Did I sign a license agreement when I purchased it? Was there some sort of "By opening this CD you agree to these terms of use" sticker on it? No. If it is simply a material item, it is no different than modifying a car.
When you buy a DVD and watch it, you will see the copyright notice which states who owns the rights. Same with a book. Its all there, its legal lawyer stuff.
I grabbed a DVD off the shelf, and here is what it says:
All rights reserved. Licensed for distribution only in the United States and Canada. Distributed by MGM Home Entertainment. WARNING Federal law provides severe civil and criminal penalties for the unauthorized reproduction, distribution or exhibition of copyrighted motion picture videotapes, laser discs and DVDs. Criminal copyright infringement is investigated by the FBI and may constitute a felony with a maximum penalty of up to five years in prison and/or a $250,000 fine. Licensed for private home exhibition only. Any public performance, copying or other use is strictly prohibited. All Rights Reserved.
You buy the DVD, you have agreed to those conditions, that is why they are there.