Um, okay …
I guess the band doesn't want to openly be known as a band that uses backing tracks, so if anyone calls them out on it they can point at you and say "look, we have a real live bassist!"?
Who recorded the backing tracks? Can you sabotage the tape deck during playback on stage? Pretty sure they'd need you then.
How long have you been in the band, and when you rehearsed for them did nobody tell you that you'd not be miked up and playing to a backing track? It seems odd that they just sprung this on you a week before a gig.
If you've no real attachment to them, where's the harm in just walking? Although I suppose there's no harm in just standing up for a couple of hrs and getting paid for it, unless you really can't be arsed, then just quit.
Mark