Giant Hogweed

Why is it banned in the US even though its already found growing wild in most states? I also read reviews for it here and except for touching it, is no different than having a tree in your yard preventing seeds from forming is easy by deadheading. I don't understand why it's banned?

Do any animals eat it, and do even the seeds cause allergic reaction?

Just because it's already in every state doesn't mean that every state wants even more of it which is what would happen if it weren't banned. Deadheading might work for the small percentage of people who will stay on top of things and ALWAYS deadhead it before it goes to seed, but the government can't assume that everyone will deadhead plants because 90% of people probably won't do it, and even the 10% who do will occasionally slip up and not get to it in time or miss a plant. And every time someone doesn't deadhead and seeds escape into a nearby field, etc that's where it gets out of control. Nobody's out in all the wild areas deadheading.

You can get some pretty nasty blisters from coming in contact with the sap, contact with eyes can cause severe injury or blindness, so it definitely doesn't seem like a plant that the government would want spreading everywhere for people to run into. http://en.wikipedia.org/wiki/Heracleum_mantegazzianum