If you were in a public park and you saw a royal person who was not on an official outing, and you had means, would you snap a photo? Of course you would, so would I. If you say you wouldn't, I doubt you.
I genuinely would not take their photo. I have a cousin who is a member of a band that isn't huge, but is big enough that he gets recognised around Canada and parts of the US. I can't remember the last time we went out to a family dinner with him without it being interrupted by a small stream of people wanting to take photos with him or get his autograph. When we know he will be attending, we have to request a private room at whatever restaurant we're meeting at. People need to be able to just relax and be left alone sometimes without being confined to a private place - especially those in a public role. There's on duty and there's off duty... and people need to respect that.