Why can my Mac charge my iPad but other computers can't?
Solution 1:
Because the iPad needs more than the 500 mA that the USB 2.0 spec says computers need to provide. Macwork has more information.
Solution 2:
Apple's newer machines (and some newer PCs I believe) have got USB ports that are able to provide more current. The following Apple KB article explain better:
- iPad: Charging the battery
- Powering Apple and third party peripherals through USB