Yes. I talked to Kevin Medina, their sound tech, after a show in April and he confirmed that the bass tracks are programmed into Cacie's MacBook. I don't blame the band; it's likely they can't reasonably afford a touring bassist.
They also played "Thread" as the song that aired on television last night. It can be seen here.
I literally thought the same thing when I saw a Ampeg cab without any of them playing bass. I do this live through an EQ/DI rack & maximizer into the PA. The results are pretty great considering it's an bass emulator. However, if you were to run the signal through a real bass cab and head like they do, you can hear the awesome results. It might feel unnatural at times since you wont get those sutle nuances from a human playing, but like CallMeTro
said, it's likely they can't afford a touring bassist.