It's a "legal concept" in the same way that every word used in a lawbook is a legal concept; the US is based on case law, and there is a large amount of precedent for this definition in particular. Again, refer to the Wikipedia for some examples.
I see this mistake a lot among programmers (also myself at some point). They (we) naturally think law is like a rulebook where you go through a decision tree and then arrive at a perfect conclusion, but the truth is that it's up to the courts to decide on the specifics. And they might disagree on those things with you, and with each other, but in the egregious cases disagreements are rare.