I suspect that OS 9 simply looks at the hardware it assumes it is running on. If it has USB compatibility, it expects to "see"/"here" a signal when something on that buss happens ("Hey! Here's a key press!" "BTW, the mouse just moved.").
I don't remember exactly when USB became standard on Mac hardware, but I think it was quite a bit after OS X came out. OS 9 would have had to have some Extensions to even recognize that the USB hardware was even present. Thus, on Start up, the Extension would note the presence of a USB port and set up a way of watching for 'messages.' But it may not (depending on exactly what that Extension did, actually test to see if anything was plugged into the port. It may not even have allowed it to be powered before it relinquished control back to the next item of the Start up process. So, even if something was using the port(s), it may not have recognized that fact. Of course, when you plug something into the port, a message is sent and the OS now 'knows' that and starts acting accordingly.
BTW, I took a nap this afternoon and dreamed all this up...
It may just be a European thing!