The main difference would be enforcement. Someone running firesheep has a chance of being detected, investigated and arrested if the crime is reported (even if that chance might be rather low).

If someone is already breaking the law they generally can't turn to law enforcement for protection unless the second crime is far worse than the first. Even if they do report it, practically the FBI is unlikely to show any interest - but may be inclined to arrest the firesheep user as they likely confessed directly to a federal officer.

It's unlikely that sending out a malformed TCP packet would ever be prosecuted in any case, if it would even be illegal, except for seriously extenuating circumstances.

Morally you'd certainly be in the clear as well imo.