Mikerho,

The schematic you posted is for a BLDC motor drive, not an AC VFD; different applications. The apparent thermistors shown in there are likely "Current Limiting" thermistors; a completely different application. In that case, they use the NTC versions because when the drive is first powered up, the NTCs are "cold" and have a high resistance, which limits the inrush. Once they heat up, the resistance drops and they are virtually out of the circuit. It's a common use of NTC themistors.