iPad not charging in USB? Says who?

Charging iPad via PC's USB?

I was syncing my iPad one day when I noticed the sign "Not Charging". That's odd. That's not a sign I'd expect from something that's plugged in. Is it natural for you? I didn't expect it, I didn't like reading it and I didn't like what it implied. To me, it implied that Apple didn't care whether I had an Apple's product. It cared whether I would purchase another Apple's product (latest MacBook Pro, I presumed) to support my current one. I didn't really expect to charge my iPad through PC, but having that warning in my face is like that iPad, or Apple, saying, "In yo' face!" I was annoyed.
A few days ago I made this discovery by accident. Left the iPad at 62% plugged in through my PC's USB overnight. Guess what? It was 100% (fully charged) the next morning. I still saw that "Not Charging" sign, though.