There will be trace amounts of propellant left even in a “burned-out” rocket. And some of those propellants are nasty enough toxins that even the trace amounts are worth worrying about.
I wasn’t really accouting for the distance the STS would have to travel, but yes, by the time the SRBs are sufficiently ‘downrange’ from VAFB to be able to impact Anaheim, the propellant would be mostly consumed. In that case, the remaining propellant could be washed out or sprayed over with an inhibitor with little concern of mass detonation or combustion, and the motor case disassembled at the field joints or cut into manageable sections and removed via crane and truck.
The polybutadiene acrylonitrile (PBAN) binder in the RSRMs is pretty benign and the fuel is just powdered aluminum, so the only thing really toxic is the ammonium perchlorate oxidizer, which is embedded in the PBAN matrix. If performing an on-site washout, that would require capturing and remediating the wastewater, which is a bit of a challenge but certainly feasible. The CTTB and HTTB binders are more toxic, and some propellants have energetic burn enhancers like nitroglycerine or HMX which are toxic and/or carcenogenic, but the RSRMs don’t use these due to the issues with personnel safety and quantity-distance limitations with such a large amount of hazard class (HC) 1.1d materials; the RSRMs are HC1.3d for that reason.
Stranger
Google Maps says Disneyland is a 190-mile drive from Vandenberg AFB. Which I believe is further than where the SRBs usually landed from KSC. So presumably the amount of propellant left is the normal amount left after a successful launch.
But I would imagine the hydrazine used to power the APU (as mentioned already) would not be fully depleted, and would pose the greatest risk.
As Stranger said, the solid propellant itself is fairly safe. Otherwise during inspection people would not stand on it without any mask or gloves:
As previously mentioned the SRBs have redundant hydrazine APUs which provide the hydraulic power to gimbal the exhaust nozzles. During the SRB phase of powered ascent most of the shuttle’s steering authority is via moving the SRB nozzles, not via gimbaling the main engines.
When you see the roll maneuver after liftoff, where the entire 4.5 million pound vehicle rotates, that’s mostly done by gimbaling the SRB nozzles. All the power for that mechanical movement comes from hydrazine-powered APUs, two on each SRB.
There’s not that much hydrazine, but (unlike the rubbery SRB propellant you can safely hold in your bare hands) it is highly toxic. A slight trace of hydrazine in the air can damage your lungs.
This is why both the shuttle and the much smaller X-37B are serviced immediately after landing by people in protective suits and masks. They use either hydrazine APUs or hydrazine reaction control thrusters, and the risk is those might leak after landing: http://sen.com/thumbs/1024x576/img/14132086451413209056.jpg
It’s unlikely the SRBs could reach the mainland since they and the shuttle would be headed out to the ocean from SLC-6, and it seems impossible they could reach Disneyland under any conditions.
However in the unlikely chance the vehicle exploded right after liftoff from SLC-6 and the SRBs (which contain their own self-destruct charges) were not detonated, then they ran wild and by some chance headed toward back toward the shore, the greatest immediate risk would be physical impact and fire.
SRBs are quite reliable but in 1986 a Titan IV SRB (about 1/2 the size of the shuttle SRBs) exploded just after takeoff from SLC-4 at Vandenberg AFB, close to SLC-6: An Error Has Occurred!
One of the most spectacular SRB failures was on a Delta 2 vehicle in 1997. The SRBs it uses are tiny relative to the shuttle SRB but it shows the potential for devastation if they fail when full of propellant and adjacent to a liquid-fueled vehicle: https://www.youtube.com/watch?v=z_aHEit-SqA
The risk in those cases is from uncontrolled flight that veers toward land soon after liftoff, not from the empty husk of an SRB landing on a populated area.
Yeah, I was primarily thinking of hydrazine, in terms of toxic propellants. There’s not much of it compared to the primary solid fuel, but it’s nasty enough that it’d probably be a bigger headache.
For the record—and thank you all for the insightful and fascinating responses, so far!—I was indeed imagining a non-polar launch from SLC-6.
(Like I said in the OP, an exceedingly poorly planned and executed mission. 
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