What is wrong with powering it via a laptop usb? please explain
Nothing, but you must stay within the limits of what usb can safely supply- <500ma, otherwise you risk damaging the computer. Absolute maximum current depends on your computer's hardware and it is really not something you want to test.
The arduino was powered using a desktop computer, not the motherboard but the case
USB power all comes from the same place- your motherboard, no matter if you your plugged into your front ports, rear ports, or top ports. Some external Hubs are powered seperately from the computer and can provide more current. In general, it is unwise to run power hungry motors off your usb because their stall current can be very large.