Wikipedia states otherwise:
SPC (Southern Pacific Communications) was only permitted to provide private lines, not switched services. When MCI Communications released Execunet, SPC took the FCC to court to get the right to offer switched services, and succeeded (the "Execunet II" decision). They decided they needed a new name to differentiate the switched voice service from SpeedFAX, and ran an internal contest to select one. The winning entry was "Sprint", an acronym for Switched PRIvate Network Telecommunications