Speaking as someone who was once in the industry, you don't have to blow up the solid rocket booster. All you have to do is 'vent' it.
The way range safety works (or at least how it worked in my day) is that there is a predetermined area that the rocket must stay in during its flight so that if all propulsion is shut off it won't land anywhere where there are people. If the propulsion is still applying thrust, then all bets are off--it could land anywhere including in a crowded city. Thus the destruct system only has to shut off the propulsion, under command when it is noticed that the rocket is heading out of the range safety area. It is burning and thrusting out one end, so blow a hole in the other end or along the side and it no longer has any directional thrust.
Solid rocket boosters get a bad rap. They did not explode during the Shuttle disaster. I repeat, they did not explode. Look at the old videos. Because of the O-ring issue, they sprung a leak of flame, which ignited the liquid fuel, which exploded. After the explosion, the now free solid rockets flew off in random directions, after which, I assume, the distruct command was initiated to vent them so that there would be no net thrust, as I explained above. They -never- exploded, even after the liquid fuel exploded right next to them--they kept right on working. Solid rocket boosters are much, much safer than liquid fuel. The only issue is that you can't shut them off. But you can vent them in an emergency which has the same affect.
