Sounds like they've incorporated a switchable transformer.
Transformers change AC to DC by the expanding and collapsing magnetic fields passing through a coil of wire. The "movement" of these magnetic fields produce a current upon the wire. Transformers are the most energy efficient devices man created, with a power factor of ~95% or better. If you were to take a transformer with a 1:10 ratio, 120V AC - 12V DC, you maintain the same voltage, but the current available has increased. If you're on a 30 Amp breaker then available DC current would be 10 times that, or 300 Amps. In that specific psu application you talked about, the transformer must have a maximum amperage rating of 80 Amps.
You can then go back and regulate the amperage rating for the 12V a few different ways. The simplest method would be a set of resistors that reduce the output current. Someone's using their head and applying Ohm's Law to the output of a power supply. I = V/R , or Current (I) = Voltage (V) / Resistance (R). Some simple Algebra leads to: V = IR, and R = V/I.
Resistors in parallel reduce current output, but not voltage. So, I'll demonstrate with a couple figures I've compiled, starting with a 40 Amp rating. Input will always be 12 volts at 80 amps, and using resistors is the simplest/cheapest method.
Reduction by 40A still maintains a 40A output.
12v / 40A = .3 ohms : Output of 12v @ 40A
12v / 30A = .4 : Output of 12v @ 50A
12v / 20A = .6 : Output of 12v @ 60A
12v / 10A = 1.2 : Output of 12v @ 70
12v / 0A = 0 or a direct output from rectifier/cap 12v @ 80A
As you can see, by including a switch to choose between output resistance, you can vary the output current. This can also be done with a single potentiometer, or rheostat. Similar to the voltage control on a dc fan, it varies the resistance to the load.
"So why don't companies produce higher amp power supplies?"
Well, to support the higher amperage ratings, it would require larger, higher rated, components, and heavier gauge wire. These components make for a larger psu, as well as, increasing the price.
Now that I've thought about how that could work, I'm slightly surprised this hasn't been introduced earlier. Use a larger transformer, and allow switchable amp ratings. I've seen the same application in power amps and such through school, but never applied it when thinking about power supplies. Quite nice indeed.