What makes a plant (Ob)noxious?

I've often heard it said that the definition of a weed is any plant growing out of place, or where it is not welcome. In that sense a dandeilion is NOT a weed if you want it for the edible parts or the pretty flowers. Clearly, people think different things when they think of Dandeilions / "Dandy Lions"

So you could say it's all a matter of perspective. A rose would be a weed if you don't happen to like roses.

However, SOME plants it seems have been deemed to be too nasty to tolerate, no matter how pretty they are or how much some individual might appreciate them...such as my now long gone "policeman's helmet". So I got to wondering, what makes a plant deemed to be dangerous enough to the local environment that it is no longer a matter of private opinion, but gets legislated as "Noxious" with state regulations demanding it be eradicated?

This is what the Idaho's Noxious Weed Pagee says:

What is a noxious weed?

Noxious weeds are plant species that have been designated "noxious" by law. The word "noxious" simply means deleterious, and all listed weeds are deleterious by definition. There are hundreds of weed species in Idaho; in 1977, 35 were designated noxious by Idaho law. [As of 2009, there are 57 noxious weed species.] The Idaho Department of Agriculture uses the following criteria for designation of a noxious weed:

It must be present in but not native to Idaho;
It must be potentially more harmful than beneficial to Idaho;
Eradication must be economically physically feasible;
The potential adverse impact of the weed must exceed the cost of control.

So there you have it. I admit, I was tempted to keep my plant and just not say anything when I discovered its bad rep. After all, it was pretty well controlled in its own little corner of my private back yard. After all, it didn't seem obnoxious to me. (I really do like the term "obnoxious weeds" better no matter what the official law calls them.)

But as corny as it may sound, for me it was a matter of integrity. I do not want to be a person who is sneaky about breaking laws. Even though I saw very little risk, eliminating the plant seemed like the right thing to do.

I will admit I HAVE done some things against the rules. I have fed ducks in a pond clearly marked DO NOT FEED THE DUCKS. I have driven faster than the posted speeding limit. I have unbuckled my seat belt before the plan had fully arrived at the gate. (oh horrors!) But for the most part, I really do try to be an honest, law abiding citizen. In most matters, I don't think it is right for me to pick and choose which rules are comfortable or convenient to follow and to ignore those that I don't happen to like. I think there is a time and place for civil disobedience, but that's for a different post.

For now I just knew I would not appreciate my sweet plant knowing it was kept surreptitiously. So out it went. Still, I will I'll miss it some. Cest la vie.