I don't think the coursework is intended to be there as busywork in case you prove the Riemann hypothesis on your first day. More likely, the faculty has made the decision that it wants to get all of its graduate students up to some minimal level of mathematical/statistical competence prior to research training, and a substantial program of graduate-level coursework achieves this.
University faculties have a great deal of discretion in deciding on the coursework component (if any) of their PhD program. Some have no coursework, and some have a substantial amount of coursework. In cases where a student comes in well-prepared (e.g., with an existing coursework Masters), the faculty might exempt them from some or all of the coursework. These decisions tend to be made at the level of each faculty, so they depend heavily on the preferences of the Head of School, the Graduate Coordinator, and other senior academics in the faculty.
I can't speak to what is common in the US, but when I did my PhD (Statistics) at ANU (Australia) there was no coursework in the degree; all the entrants had either done a full year of Honours-level courses as undergraduates, or a Masters degree, or they had substantial industry experience, so they came in with a fair bit of solid coursework behind them already. Evidently, in that particular case, the senior staff in the faculty decided they did not need us to have any more coursework before starting research training.