There's different markets of Computers.
Smartphones, Tablets, and PC's are the main ones.
PC's have different markets as well. Desktops, Laptops and Netbooks.
A PC Vendor makes PC's. Tablets are not PC's. Computers yes, PC's no.
If you think the iPad is a PC, then Why not add in the iPod Touch? It's a smaller version of the iPad? Why not add the iPhone? It's a smaller version of an iPad with phone capabilities. They are running the same software.
If you do, you might as well throw in all the smartphone devices and call them PC's.
Why stop there? Symbian phones can run applications. They have CPUs, GPUs, RAM.
BREAKING NEWS: NOKIA BECOMES TOP PC VENDOR
This is why the iPad isn't a PC.
It depends on consumer perception and the way devices are used by the average person, not contrived definitions that reinforce your comfort with your own world-view.
Joe Average doesn't consider smartphones PCs. He does, however, use an iPad like a PC. End of story. You don't have to like it. But it is why it is. Canalys is simply recognizing the trend. They aren't trying to provide a psychological/technical explanation. A lot of people in this thread seem to be begging for one, and as a result are tripping over their own words and concepts in order to keep their heads from exploding.
Change and the uncertainty in trusted, long-held assumptions it brings with it can be frightening.