The PSU does control the output, but the interface controls the PSU. There are several ways to run an argon like this. The first is simply to jump the pins needed to run it at "idle." Idle in an argon works like idle in a car. The laser is lasing, but at a low level. The second way is in "light mode." In light mode the PSU gets feedback from a photodetector in the laser. This automatically runs the laser at mid range. The last way is to run it in "current mode." In current mode the PSU is intentionally adjusted to increase or decrease the output of the laser. Idle and light mode can be achieved by jumping pins. In order for current mode to operate, there must be a potentiometer in the circuit.
I will ship the laser with the pins jumped for idle. That way the buyer can have argon goodness with the least trouble possible. In order to run it at higher outputs, the buyer will have to do some limited soldering and get a potentiometer (or simply buy a remote interface to do the job).
If you can be bothered you can build a remote interface. I've built one for mines which consists of a " borrowed " 25pin connector, a metal case with pot and switch and an analogue multi-meter taped onto the metal box. This works fine for me, but if you want something better, or more professional, you could have a look on eBay.