It is not illegal because my friend went there and told me it was great. It is a bit dangerous for this part is not as neat as Badaling which was preserved very well. Jiankou is a part of 'wild Great Wall' favored by more and more backpackers. Be cautious while traveling there.

Like a lot of things in China the idea legality is fluid. Hiking on the wall in Jiankou may be 'illegal' but it is not policed and you are sure to share the experience with many locals (and a few expats). However it is dangerous. My local Chinese friend said that a few people have died in the area after slipping of the wall in wet weather. Also in parts you must rely on a rickety ladder and bridges/walkways. In terms of getting there it is not easy. The way I got there was with a Chinese friend. We took a public bus headed for Mintianyu (can't remember bus number) jumped off a the roundabout and convinced some people to drive us to the starting point (near the trout farm) we hiked and ended up back at the trout farm and convinced some locals to take us back to the bus stop (which they did for free (generally Chinese people are very generous).
I hope to go again soon, because it is honestly stunning, with the true feeling that you are walking on something that is truly old and great.