A few years before this, when Lily was at Hogwarts, other students made fun of zem for focusing on the study of Ancient Runes instead of something more useful for self-defense, like Charms or Transfiguration.

The Ministry prohibits the possession of books of “dark magic”, such as the one Lily got that spell from, without special authorization. The Hogwarts library has this authorization, but Lily does not. However, at the height of Voldemort's power, the Department of Magical Law Enforcement has more important things to do than bust people for having banned books. The law mainly functions as an excuse to make arrests if Ministry officials suspect someone of being a Death Eater and raid zir home, but don't find real evidence. Many people have such books out of curiosity, or for self-defense, or because a few “dark” spells were included in a book about something else. However, once it's gotten to that point, DMLE agents don't stop to wonder if the accused might be innocent.