I used to (like, 20 years ago or so) use fire rings, but have never built one.
I guess I've broken some up, but it wasn't so much in the attitude of destroying it as it was making things look a bit more "natural" - especially if was crudely made, recently made, or long since "abandoned."
To echo what some people have said - if you do it right, you don't need the ring. Though, if done right, it makes a good break alongside all the other precautions (clearing duff, etc). Luckily, I was trained properly as a child, and can make small, hot, purposeful fires that when extinguished, are easily covered/spread/removed.
But since getting back into nature, I've abandoned all usage of fire. It's just not worth it. Sure it glows, it warms, it soothes some ancient emotion....but its just too dangerous now. And after last year? Forget it. I still take all my emergency fire-starting equipment, but it's for just that - emergencies.
I'll admit that my first fireless trip was kind of a bummer. Something was "missing." But since then, it's fine. I'll live. The only real effect is that I don't have much of a desire for cooking fish in the backcountry anymore. Smokey, peppery trout was such a treat, it just doesn't have that same "flavor."
So, as for illegal - it's pretty much anything now. Folks can easily go without now that there are amazing lamps and stoves and water filters.
I think I went nowhere with all this.