If you are an experienced rider this is actually annoying as hell. Personally, I tend to leave whatever I'm riding in a high gear all the time, because my body is like a powerful machine, and I found the way it slowed my escape from the lights quite disconcerting. For beginners, it could be useful, but it's worth remembering that the whole point of e-bikes is that the motor helps you along anyway, so I do really question the usefulness of this.
The federal Consumer Product Safety Act defines a "low speed electric bicycle" as a two or three wheeled vehicle with fully operable pedals, a top speed when powered solely by the motor under 20 mph (32 km/h) and an electric motor that produces less than 750 W (1.01 hp). The Act authorizes the Consumer Product Safety Commission to protect people who ride low-speed electric vehicles by issuing necessary safety regulations. The rules for e-bikes on public roads, sidewalks, and pathways are under state jurisdiction, and vary.