Well,
I'd have to say "that is how it works".
The Line 6 hardware wants to do the in and the out,
and it is my impression that most of the other interfaces work this way too.
That doesn't mean you can't change it though.
There are preferences in Garageband that allow you to set the audio inputs and outputs and then there is a window at the bottom, that has settings for monitor. (on with feedback protection works)
You might get some feedback warnings or some other strange results, but there is no reason not to do this.
But you have to set this. The GX will assume in and out when you use it. They say to plug your "monitor speaker" set-up into the line out of the Line 6 hardware instead of into your machine, but I know that is impractical for most.
Start here:
http://line6.com/data/l/0a060072fbd6490f140ce227e/application/pdf/POD%20Farm%20U ser%20Guide%20(Rev%20D)%20-%20English.pdf
POD Studio, TonePort and GuitarPort hardware - Input sources fed into the Line 6 hardware are handled by the audio driver, which manages applying the POD Farm Tone processing to your signal, feeding the processed audio out the Record Sends to your audio software, gathering playback audio from your audio software and then routing the audio back to the Line 6 hardware’s outputs and to your monitoring system. The audio driver also grabs the POD Farm processed signal before routing it to the Record Sends and hands this off to ToneDirect™ Monitoring immediately, to provide a low latency monitor signal, which is then routed to the Line 6 hardware and mixed with the rest of your audio and fed to your monitoring system.
Translation: plug the line 6 hardware into your stereo or other powered speaker system of some kind.
But you don't have to. I would describe the setting of the Garageband Prefs better but I can't start it right now. (pumping video)
