most dangerous power of the prosecutor: that he will pick people he thinks he should get, rather than pick cases that need to be prosecuted.” Prosecutors could easily fall prey to the temptation of “picking the man, and then searching the law books...to pin some offense on him.” In short, prosecutors’ discretion to charge—or not to charge—individuals with crimes is a tremendous power, amplified by the large number of laws on the books.
The messy state of US law, with many preposterous and outdated laws on the books, encourages this kind of discretionary enforcement. Even police officers can often "pick the man, then figure out the crime."