I was curious what the difference was so I went searching for a study like this one and saw on Table 2 that voluntarily admitted treated individuals at the 6 month follow-up survey were only 50% abstinent for the prior 30 days compared to 24% of compulsorily admitted. However, it is true that the overdose rate for VA was much lower than CA's 22%.
I'm not sure courts anywhere in the USA can force rehab for more than a year, some states do about 30-50 days. Even then, usually only for repeat offenders. With that in mind the number of people who complete treatment for compulsory admission is probably much much lower than for VA, but I didn't bring stats to back that claim up.
I do also know for a fact, though, that the 2 year rate is even worse than those numbers, so rehab is more often unsuccessful regardless.