I do not believe I have ever seen such a sign before:

Odd sign, until you realize it’s difficult to fly a kite in a canyon and avoid the power lines. Photo from Poky Tom’s Flickr files, Thousand Springs, Idaho.
About 1982 I bought a couple of kites and string and kept them in my office on Capitol Hill. I hoped someone would some day tell me to “go fly a kite,” whereupon I would announce that’s exactly the thing to do, grab the kites and rush to the Washington Mall to fly them. (Do they allow that stuff, there, anymore?)
Alas, none of our pitched battles over policy and press release phrasing got to that point. The kites got lost in the move from Maryland.
I came up on this photo, and the explanation tickled. You may see why. Poky Tom wrote:
Grounded at Thousand Springs!
Today, the first day of World Wide Kite Aerial Photography (KAP) Week 2011, I was excited to finally end my 3-year jinx of getting skunked during WWKW. The weather was good with reasonable wind. We knew the Thousand Springs area would be great for photography. We pulled into the parking lot, which is shared by the Thousand Springs State Park and the Idaho Power hydro power facility. I got out of the car and was immediately confronted by this sign. Curses! Move on.
I’ve never heard of kite aerial photography as something almost organized. Poky Tom has some wonderful shots from a kite, though. He also uses a 30 foot pole to get great results.
But, no, you can’t fly a kite there.
More:
Spread the word; friends don't allow friends to repeat history.