If the Voluval is 99.9% valid, that means that you only miss one sociopath per thousand sociopaths, but if you only have one sociopath per 1000 people, then for every sociopath you detect, you probably get a false positive on an innocent person. (note that these numbers are wild guesses to illustrate the situation, assumes only two options, and that the answer is a yes/no option; and therefore should be taken with a large dose of salt)
So, even with 99.9% valid detectors, half your exiles will be (more or less) innocent.
So sure, there's a level of unfairness on a bunch of the exiles, but I imagine that such a level of accuracy might still be considered acceptable to a lot of people in the tribes.