Why would a digipot require the voltages on its terminals to be between ground and VCC?
This device is far more than "just a resistor ladder". It includes a great deal of digital logic that controls the serial interface and allows you to program the resistance.
The resistors are connected to the "wiper" by analog switches. These switches are made of transistors. There are parasitic PN junctions between the transistor terminals and the power and ground terminals. These PN junctions remain reverse-biased, as desired, only as long as the internal voltages remain between Vss and Vdd.