By your logic, hand written or verbal threats shouldn't be a crime either because they too can be altered.
Sorry to hear that you don't post at D.U. for this reason, I have better reasons.
No, the alteration and in some cases, fabrication, of a hand-written or typed statement can be verified in a court of law. If it is found to be an original, then it stands as evidence. [If you don’t believe me go ask Dan Rather. :] This is established case law.
On the other hand, there is literally no way that a web posting can be proven to be in the same state as made by the original poster. As soon as you click “Post”, it could be altered.
The new law is an attempt to extend existing case law into a very fuzzy area. Unfortunately, it is fuzzier than the folks who write the laws will understand. Hopefully, some sharp judge will temper this with some sense.