Doesn’t the Constitution override British case law?
No, not really. The Constitution marks a formal separation between England and the United States, but part of being a common law nation is that past cases always continue to have some influence on the jurisprudence of the presence. The legal system of the early Republic was largely derived from English common law, and that body of law continues to provide a basic structure for our legal system.
At any rate, from a cursory look at the history here, I'm not sure why you're assuming that the Constitution prohibits the practice — forfeiture was a practice under British statute (not case law) and one of the early Congresses almost immediately passed a similar statute.
