It's different because a picket line can be crossed. Picketing relies on convincing potential customers to choose not to patronize a particular business. A better analogy for a DDOS attack might be deliberately blocking the doors so customers can't get in--for which the business can (and often successfully does) sue for lost income.
This isn't to say that picketing doesn't sometimes get out of hand, or that the penalty currently on the table isn't way too high. To be honest, I always thought that these sorts of damages were handled in a civil lawsuit after the criminal proceedings. But I'm not an expert in law.