SAR & HEMS operate in a time critical environment. An onlooker has no idea what is planned. If the crew is strapped in, engines and/or rotor blades are turning then this surely means they are planning to do something, or may be.
The needs of an onlooker to rubberneck have a very low priority. If the crew are hanging around and all is still, then it may be reasonable to ask. Otherwise for heaven's sake give them a break, keep clear and let them get on with their job.