New U.S. citizens must swear the following oath of loyalty to the United States (quote from https://www.uscis.gov/us-citizenship/naturalization-test/naturalization-oath-allegiance-united-states-america):
I hereby declare, on oath, that I absolutely and entirely renounce and abjure all allegiance and fidelity to any foreign prince, potentate, state, or sovereignty, of whom or which I have heretofore been a subject or citizen; that I will support and defend the Constitution and laws of the United States of America against all enemies, foreign and domestic; that I will bear true faith and allegiance to the same; that I will bear arms on behalf of the United States when required by the law; that I will perform noncombatant service in the Armed Forces of the United States when required by the law; that I will perform work of national importance under civilian direction when required by the law; and that I take this obligation freely, without any mental reservation or purpose of evasion; so help me God. (Emphasis added.)
Why does the oath of loyalty to the United States that new U.S. citizens must swear mention seem to include renouncing all foreign citizenships, since in practice the United States allows their citizens to hold several citizenships?