Look, it can be summed up pretty succinctly: the premise behind US laws is that if it isn't explicitly forbidden, then it is allowed. Thus the laws like this one, which expand protections provided elsewhere into the online world.
Oh, so there isn't an offline protection against impersonation? But of course there is. Most states make it a crime to, say, call someone's current employer and pretend to be that person's former boss. It's not mere impersonators that is forbidden, but impostors. A small difference, but an important one. It's the difference between actors impersonating a real person in a play or movie, and an actor impersonating a real person outside of that setting, not providing clues that it is just an impersonation.
Or another example: if your name is Dan Brown, as long as you don't try to convince people that you're the famous author of The Da Vinci Code (assuming you aren't him, of course), then it isn't impersonation. As long as you don't try to convince people you're one of the other JW Smythes out there, you aren't an impersonator. Just a homonym.
Oh, and as to why some things are forbidden (like texting while driving) and others aren't (doing X while driving), it goes back to the old adage of "what isn't explicitly forbidden". Until enough incidents occur where, say, leg shaving played a role in an accident, it won't be explicitly forbidden. It will be assumed enough people know better to actually make it illegal.