Most of the ones I've seen are the keyboard-injection type devices, because that's the simplest way of doing it - no drivers necessary.
Now the wikipedia article does mention that a few of these devices do have RS232 interfaces. But the device still needs to get it power from somewhere.
The PS/2 interface has a 5 Volt supply, so that's the most probable source. But older, RS232-only devices typically scavenged their power from the RS232 interface itself.
But, those devices may not be able to derive enough power from a 'non-strict' RS232 interface - RS232 is suppose to be +/- 12 Volt signaling. But a lot of 'adapters' have 'customised' this standard, using much lower voltages, or not even using