Well, it is a harm from their perspective. I'm just reporting their perspective. And it's true that extensive night lighting makes their job more difficult.
I happen to own a couple telescopes.
Let me ask you. Would you welcome all night loud parties next door to your home? No, it would interfere with your right to enjoy your property, and that is a reasonable purpose to have noise regulations.
I don't welcome all night bright lights next door to my property, because it interferes with my right to enjoy my property. No difference.
If you keep your light to yourself, have at it. Just don't shine it in the direction of my telescopes.