Second, the boosters cannot be shut off. That's the big safety drawback of solid rockets - you light them, and they aren't going out until they're out of fuel.
*sigh* This is one of the biggest pieces of misinformation about solid rockets floating about out there, spread and repeated by shuttle detractors in a cargo cult like fashion until it's now regarded as a law of nature. What most people (including engineers who should know better) don't realize is that you don't need to shut them down in the first place- you just need them to produce net zero thrust. This is done via blowout panels in the front dome, and sometimes by blowing off the nozzle as well. And it's not like this is a new fangled technique either...
Actually, you can do even better!
It has been known for many decades that you can quench a burning propellant by subjecting it to a rapid pressure decrease. (The conductive flame structure cannot rapidly adapt the the decrease pressure and goes out.) Thus, blowout panels could actually be designed to quench the solid boosters. And this knowledge existed when the shuttle was designed.
But there is a finite price on human life and, like the ejection seats or parachutes on the shuttle or passenger airplanes, losing a few dozen astronauts is cheaper than accommodating safety systems.
We make it seem like it would be impossible to have these safety systems, but it isn't impossible. It would just be less efficient for the company or government... who only really cares how much your death costs them.