It's not a PC, it's a mobile computer.
User-programmability is what separates a personal computer from a content-consumption device. You wouldn't call a Wii a PC, or a Roku box a PC, so you shouldn't call a non-user-programmable tablet a PC.
I would say the the PC is a device that tailored toward the production of content and information. The tablet is tailored more towards the consumption of content and information. Certainly the tablet and the PC can do both production and consumption, but the PC is better and production than the tablet is.
Everything about the PC, from the capabilities of the processor to the layout of the hardware and software, is designed to make the user more productive. I can't imagine using a tablet to write software, or a novel or technical manual, or do CAD or design work on a tablet, or even just the day-to-day tasks of a 9-5 office worker.
The tablet, on while it might be great for short bursts of production (e-mails, notes, etc), is better at providing instant access to information. It is a device or viewing and using what people with PCs produce.
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