You can design the pushbuttons to pull ground two different ways. You can put a resistor between 5v and the pin on the microcontroller of the IPAC and then close a switch to ground when the button is pushed (how the IPAC is built).
Or you can put a resistor between ground and the pin of the IPAC, and have a normally closed (NC) switch between that and 5v, and have the switch open when you push the button.
Or you can do either of those, switching 5v and ground, for a circuit that has active-high inputs.
I also posted some circuits that use both the N-O and N-C pins of the microswitches, connecting those to ground and 5v respectively, with the common pin connected to the circuit.
Basically it's because there are uses for both types of switch operation, and the way switches are made there's really no good reason not to have it there.