The issue isn't limiting use to just the company's chargers, but complying with the USB spec.
A lot of devices just draw power from a USB port irrespective of what's happing on the data lines. This is not following the USB spec, and is basically anarchy. If you plug a device into a charger with floating data lines, the device pullups will usually cause a J state to be seen on the bus. (What happens to the non pulled up line is not entirely certain, but is usually weakly pulled down.) The spec will tell you that a continuous J state on the bus is suspend. The spec also tells you you're only allowed 2.5µA of power in suspend, which is zero power for all intents and purposes.
We care about being spec compliant, so you can't just say if you plug into a random USB port (which you don't know if its a charger or sleeping computer or what), just draw 500mA. The spec is telling you that the power is not available.
Apple's charger, and later the USB-if's charger scheme were hatched to make it easy to create a charger which communicated enough information to the device to know that it was attached to a charger port, so the device could charge. In Apple's case you need 4x1% resistors, which some manufacturers think is too much hardware to put in a charger. In the USB-if's case, all you need is a short. Just short the data lines on your charger and your device should know that it's safe to charge from that port. No spec violations needed. I even made my self a cable for one device which had the data lines shorted, that way I could plug the device into any port that I found (which I knew was capable of sustaining the charging current), and the device would charge.
If you plugged a USB-if compliant charge capable device into an Apple charger port, it wouldn't see a charger port, so probably wouldn't charge. I would not expect any harm to come of it. (You could use a shorted cable like I did, it worked with Apple chargers.) If you plug a dumb device (which doesn't check for charger ports) into an Apple charger it should just charge. The Apple chargers are quite well designed and make a nice compact chargers.
The problem is there wasn't a standard extant when Apple faced the problem, and the USB-if used a different method when they did specify charger ports. So now you have two incompatible standards.
