I would have agreed with you up to about ten years ago when I began to hear of them. When I was a younger man working in LE there were none. The oath a new citizen took required they forsake their previous citizenship. But we have dual citizens now. When, why and how it came about I have no idea.
The law must have changed then. The last person I talked to about dual citizenship (not that long ago!) told me that US law did NOT recognize it. You forsake all other allegiances as you said. I’d like to see a link to the new law.