Because the consequences of a screw-up are much more significant in a car, than on a bike. Bikes travel much more slowly than cars, have much less mass, and cyclists have much more spatial awareness than drivers (they're not surrounded by sound-proofing metal and glass).

What we're discussing is a minor change to the vehicle codes, to permit bikes to treat stop signs as yield signs, and red lights like stop signs. This is already the law in Idaho, and AFAIK there's been no problems with increased accident rates...and the drivers and cyclists of Idaho seem to be able to deal with the differences without undue confusion.

FWIW, here are the relevant sections of the Idaho traffic code:

49-720. STOPPING -- TURN AND STOP SIGNALS.

1. A person operating a bicycle or human-powered vehicle approaching a stop sign shall slow down and, if required for safety, stop before entering the intersection. After slowing to a reasonable speed or stopping, the person shall yield the right-of-way to any vehicle in the intersection or approaching on another highway so closely as to constitute an immediate hazard during the time the person is moving across or within the intersection or junction of highways, except that a person after slowing to a reasonable speed and yielding the right-of-way if required, may cautiously make a turn or proceed through the intersection without stopping.

2. A person operating a bicycle or human-powered vehicle approaching a steady red traffic-control signal shall stop before entering the intersection, except that a person after slowing to a reasonable speed and yielding the right-of-way if required, may cautiously make a right-hand turn without stopping or may cautiously make a left-hand turn onto a one-way highway without stopping.

In other words, cyclists can roll through stop signs if there is no other traffic present. And cyclists can ride through red lights, after stopping, if there's no other traffic present.

IMO, these are good laws - they would help to encourage cycling for transportation, and would decriminalize what is already common behavior.

They would also confer a degree of "specialness" on cyclists...something the VC crowd seems to take issue with. But, given the many benefits to society of cycling, I think that granting them some special rights in traffic is warranted and justified.