As with most things in life, it's not black-and-white... both are happening (although fleet vehicles tend to be more visible at the municipal level). You do still need national-level policy to set market certainty and reduce risk, though (a car maker is much more likely to invest in significant retooling if they're guaranteed a market for that retooling).

Even in the US, the USPS is experimenting with electric mail trucks: https://about.usps.com/what-we-are-doing/green/vehicles.htm

If the ban is the stick, the best return on the carrots would be increasing public charging infrastructure. SF runs chargers in front of city hall, for example.