One problem with the nothing-to-hide argument is that it assumes innocent people will be exonerated certainly and effortlessly. That is, it assumes that there are no errors, or if there are, they are resolved quickly and easily.
Suppose the probability of a correctly analyzing an email or phone call is not 100% but 99.99%. In other words, there’s one chance in 10,000 of an innocent message being incriminating. Imagine authorities analyzing one message each from 300,000,000 people, roughly the population of the United States. Then around 30,000 innocent people will have some ‘splaining to do. They will have to interrupt their dinner to answer questions from an agent knocking on their door, or maybe they’ll spend a few weeks in custody. If the legal system is 99.99% reliable, then three of them will go to prison.
Now suppose false positives are really rare, one in a million. If you analyze 100 messages from each person rather than just one, you’re approximately back to the scenario above.
Scientists call indiscriminately looking through large amounts of data “a fishing expedition” or “data dredging.” One way to mitigate the problem of massive false positives from data dredging is to demand a hypothesis: before you look through the data, say what you’re hoping to prove and why you think it’s plausible.
The legal analog of a plausible hypothesis is a search warrant. In statistical terms, “probable cause” is a judge’s estimation that the prior probability of a hypothesis is moderately high. Requiring scientists to have a hypothesis and requiring law enforcement to have a search warrant both dramatically reduce the number of false positives.