IP lawyer here :)
"If so, why can't an author make this part of the license?"
So let's separate out two questions implicit here:
Can you make this part of a license?
Would you win if you sued someone for violating it?
The answer to the first is clearly yes, you can license it however you want :-).
However, like most IP, in basically all countries there are limitations on how you are allowed to license things, to ensure the original goals of copyright are respected.
Those limitations are usually presented as defenses.
Van gave two examples of defenses.
There are others.
For example, patent misuse is a defense to patent infringement.
Here's a concrete example of patent misuse that may be easier to understand than copyright misuse:
You sell a thing that infringes my patent.
In order for you to avoid infringing my patent, I force you to pay royalties for 10 years (Rather than a smaller length of time).
But wait, the patent expires in 5 years!
Can I enforce this?
No. It's patent misuse. You cannot use your currently valid patent to force someone to pay royalties past the validity period of your patent.
Another example:
You have a product X that infringes my patent.
I use my patent on X force you into an agreement to pay royalties on a product Y that doesn't infringe my patent.
Can i enforce this?
No. You shouldn't be able to use the patent on X to force me to pay royalties on something that doesn't use your patent.
To bring it back to Van's examples, the copyright misuse is similar to the patent misuse i just gave you.
You can't leverage your copyright on X to force me to do something with an unrelated thing.
The GPL and AGPL are very careful about this. The AGPL applies to modified versions (which are derivative works) and only extends to pieces that are derivative works.
The GPL is the same - only the derivative works are touched.
That is within the copyright rights of the thing that was AGPL/GPL'd.
How do you know it's unrelated to a given copyright?
If X could not claim any copyright rights over it, it's unrelated. This usually comes down to whether it's a derivative work not because the other rights are not very broad in coverage.
Here, the license is taking X, and saying "You must do something with unrelated thing Y, which is clearly not within the scope of copyright of X". So I am trying to use my copyright right in X to force you to do it.
Note: No lawyer believes there is any reasonable argument that completely and totally independent piece of software X that does say, monitoring, is a derivative work.
So that's a good start on copyright misuse :)
(The other prong is about whether it restricts competition, which they already admit is their goal here)