This is easily reconciled if you distinguish between the public officials doing the selective enforcing, and private citizens selectively deciding who to do research on. Let's say 20% of waiters at a restaurant under-report their tips to the IRS. It would be objectionable if the IRS decided to target one guy in particular because the agent had a grudge on him, but it's far less objectionable if the IRS acted on an anonymous report that the guy was evading taxes. If anything, it would be more objectionable if the IRS didn't act, and said "well 20% of waiters are evaders as well, so it would be unfair to investigate that guy because we don't investigate the rest".