sorry, should say "carry", not "buy". most states have a maximum length you can carry (4-5.5 inches is common).
although, i would imagine at some length, it becomes a "sword" (even if marketed as a knife) and falls under some other "nanny"-ing. i have not googled that.
As kevin_thibedeau points out elsewhere in the thread, he's not necessarily wrong. In many states and foreign countries it's illegal to carry a large knife in public without a reason and I'm sure purchases are restricted in some places as well. Most people are more or less OK with that, it seems, so there historically hasn't been a lot of pushback.
although, i would imagine at some length, it becomes a "sword" (even if marketed as a knife) and falls under some other "nanny"-ing. i have not googled that.