Hmm. I'm going to make a guess that before this was announced to the world, someone, somewhere was asked to take a look at it and carry out a bit of a risk assesment.
I'd also guess that that risk assessment determined that no-one's really out to get the royals. Not saying there aren't people out there who'd take a pop but there maybe aren't any known plots.
It also probably considered that Air Ambulance attendance is, pretty much by definition, in response to random unplanned events.
You can't just ring up and ask for the Air Ambo. It's requested by first responders so it'd take a determined effort by a volunteer to be ill or injured enough to trick its attendance.
They perhaps came up with the conclusion that, on balance, HRH flying a helicopter to unknown locations at unscheduled times is less risky than cycling in London. Or playing polo. Or pretty much anything.
