It's not that uncommon for citizenship changes, in either direction, to be public record. In some countries they're even pro-forma passed as laws through parliament, which of course then becomes public record. (In Denmark, at least, only the parliament has the right to grant or revoke Danish citizenship for any reason, an old restriction intended as a protection against the King arbitrarily doing so. In practice the immigration authorities draw up lists of people, and Parliament rubber-stamps the lists a few times a year, reading them into the legislative record.)
Another reason to make them public is that otherwise there's no way to determine if someone is actually a U.S. citizen, since there's no central register. For example, someone with a birth certificate showing they were born in the U.S. is a U.S. citizen by birth... unless they've renounced it. So to determine if someone is a citizen you need: 1) a copy of their birth certificate; and 2) a list of renunciations so you can check that they aren't on it. Sort of like how key revocation works in crypto— you can't actually haul back the original documents, so you have to publish a revocation list.
Another reason to make them public is that otherwise there's no way to determine if someone is actually a U.S. citizen, since there's no central register. For example, someone with a birth certificate showing they were born in the U.S. is a U.S. citizen by birth... unless they've renounced it. So to determine if someone is a citizen you need: 1) a copy of their birth certificate; and 2) a list of renunciations so you can check that they aren't on it. Sort of like how key revocation works in crypto— you can't actually haul back the original documents, so you have to publish a revocation list.