Vandalism in California requires "maliciously damaging, destroying or defacing someone else's property". A cone on a hood does none of those things.

It might be some other crime, but we can't slap people with charges that won't stick in a court of law.

The counterpoint to the argument is that you shouldn't be deploying deadly machinery on public roads with no ability to handle common issues and expecting it to be a problem for the SFPD/CHP.