DDoSing is already an illegal cybercrime in the US. A DDoS attack could be classified as a federal criminal offense under the Computer Fraud and Abuse Act (CFAA)[1].
Under Damaging a Computer, 1030(a)(5)
> The provision may also be used to prosecute the perpetrators of Distributed Denial of Service (DDoS) attacks, which occur, for example, when an attacker overwhelms a server’s ability to process legitimate requests by overloading the server with a flood of illegitimate traffic.