> Linux is able to run proprietary userspace software. Even most open source zealots agree that this is necessary. Why are all drivers expected to use the GPL?
To answer your excellent question (and ignore the somewhat unfortunate slam on people who seem to differ with your way of thinking), it is an intentional goal of software freedom. The idea of a free software license is to allow people to obtain a license to the software if they agree not to distribute changes to that software in such a way so that downstream users have less options than they would with the original software.
Some people are at odds with the options available with licenses like the GPL. Some think they are too restrictive. Some think they are too permissive. Some think they are just right. With respect to you question, it's neither here nor there if the GPL is hitting a sweet spot or not. What's important is that the original author has decided that it did and has chosen the license. I don't imagine that you intend to argue that a person should not be able to choose the license that is best for them, so I'll just leave it at that.
The root of the question is "What determines a change to the software". Is it if we modify the original code? What if we add code? What if we add a completely new file to the code? What if we add a completely new library and simply link it to the code? What if we interact with a module system at runtime and link to the code that way?
The answers to these questions are not well defined. Some of them have been tested in court, while others have not. There are many opinions on which of these constitutes changing of the original software. These opinions vary wildly, but we won't get a definitive answer until the issues are brought up in court.
Before that time period, as a third party who wishes to interact with the software, you have a few choices. You can simply take your chances and do whatever you want. You might be sued by someone who has standing to sue. You might win the case even if you are sued. It's a risk. In some cases the risk is higher than others (probably roughly ordered in the way I ordered the questions).
Another possibility is that you can follow the intent of the original author. You can ask them, "How do you define changing of the software". You may agree with their ideas or not, but it is a completely valid course of action to choose to follow their intent regardless of your opinion.
Your question is: why are all drivers expected to use the GPL? The answer is because drivers are considered by the author to be an extension of the software and hence to be covered by the same license. You are absolutely free to disagree, but it will not change the original author's opinion. You are also able to decide not to abide by the author's opinion. This may open you up to the risk of being sued. Or it may not.
Now, the question unasked is probably the more interesting question. Why does Linus want the drivers to be considered an extension of the original software? I think the answer is that he sees more advantages in the way people interact in that system than disadvantages. There are certainly disadvantages and things that we currently can't use, but for many people this is not a massive hardship. I think the question you might want to put to him is, what advantages have you realised over the years from maintaining the license boundaries as they are? I don't actually know the answer to this question, but would be very interested to hear Linus's opinion.