California prohibits pricing based on driving habits like speed. Other states do permit it and Progressive offers snapshot device which does what you describe - offer discounts for safe driving behavior.
Wow really, that is amazing and unsurprising at once. I can only imagine the debate that led to this regulation.
75 mph is about the average highway speed in LA - assuming you're not in stop-and-go traffic of course. Speed limits seem to be more of a recommendation than an absolute.
Their fee structure is a base rate plus a fixed rate per mile. From what I remember, those rates are determined by the typical demographic info + driving record/past claims. There are other companies that I believe try to adjust their rate based on driver behavior data, but metromile doesn't (at this time).