Police Officers are quite often either on a call (and most calls wont involve the lightbar or sirens) and are therefor not willing to stop what they are called to do; to stop someone on a Motorcycle or doing a Burnout or whatever...

The other thing that happends, is their shift may be ending and they want to finish up some reports and go home.

Or in that officers case, I bet he just decided to let Darwin take it's course. I've had many an officer just watch me zip past them at over the speed limit, simply because they more or less just feel like I'm not going to hrut anyone but myself in a motorcycle accident, so there's no need to stop me.