That's a totally different situation.

A USB port outputs 5V across one pin with another as ground. By cutting off the end of a USB cable, and splicing in a PSP charger cable to the right wires, you're now outputting 5V. Assuming the PSP runs off of 5V, then you're simply using the USB port as a power supply.

Likewise, you can reverse the process and charge a device that charges by USB by using a DC power supply (6V or greater) and a 5 volt regulator (about $2 from RadioShack), and the opposite end of a USB cable.

Totally unrelated to the PS2 controller port.