From the manual:
In the Settings dialog, you can first configure whether USB is available in the guest at all, and in addition also optionally enable the USB 2.0 (EHCI) controller for the guest. If so, you can determine in detail which devices are available. For this, you must create so-called "filters" by specifying certain properties of the USB device.
... you can define a filter for USB, but you need not. I never defined any filter for my USB sticks - simply plug in and they will be recognized.