There are numerous entirely legitimate reasons to turn private browsing on or opt to clear history. For example, many people prefer to research medical problems privately to protect their privacy. Young people looking for sexual health resources often clear their history to avoid attracting attention from their parents, as do those looking for help with sexual or physical abuse. A partner buying a present for someone might want to avoid ruining the surprise.
Enumerating reasons for the legitimate usage of private browsing or deleted records, however, is beside the point: Privacy is a fundamental right in the United States, and people don’t need to justify how they use their computers. If instructed to preserve anything that could be considered evidence in a federal investigation, people are effectively backed into a corner, guilty until proven innocent. Individuals are not evidence preservation specialists, and they shouldn’t be held responsible for clinging to every possible scrap of data that might someday be useful in federal cases.