To expand on Spiff's answer, Apple has a support document that mentions this towards the end of document.
Time Machine support for USB disks
OS X Time Machine supports compatible unencrypted USB disks connected to AirPort Time Capsule (802.11n and 802.11ac), and AirPort Extreme (802.11ac)."
So, it appears you have an AirPort Extreme 802.11n (not supported), and not an 802.11ac (which would be supported). (I would recommend updating the question to specify which model you have, for added clarity.)
When reading the support document don't be confused by the quote below that is towards the top of the document (it tripped me up at first). It mentions journaling support, but is only in reference to attaching USB devices in general, and does not include TimeMachine support:
Mac OS Extended (HFS-plus) - Formatting a storage device as Mac OS Extended with journaling is recommended, because it may provide additional resilience if the device is removed or powered down while in use.
So we can see according to these two quotes that you can use the drive as a normal network attached drive with the base station that you have with journaling enabled, as long as you don't use TimeMachine, which is consistent with your experience. But For TimeMachine to work with journaling you'd need a Time Capsule or AirPort Extreme 802.11ac.