With that level of visibility ("down to an invoice"), it's easy for people to take up pitch forks and begin lambasting teachers, for example, for their spending on computer software, materials for class, and so on. Perhaps a doctor's medical decisions would start being questioned left, right, and centre, yet the people doing the questioning have no medical experience or understanding of the patient's needs.
Let me put this another way: do you like being micro managed at your job? Would you like every decision you make regarding public spending, as an individual teacher, to be disputed, criticised and thrown up on Twitter by someone who, for example, doesn't agree with your purchasing of materials to teach a religious class, or a class on evolution, or perhaps a subject you don't think should be taught, like film & television?
I think having the data that's "down to an invoice" available to the state is valuable, but accessible to the public? No. I couldn't get behind that.
EDIT: "if" -> "is"; "ZERO" -> "no"