Of course it is not illegal, but the Smarts continue to release tidbits without any followup information, such as when the sketch was done, who did it and why aren't the police releasing it. Whenever he gets questioned about things, he says he can't answer. Yet he keeps bringing out new stuff all the time that may have helped if released earlier.
We can conjecture what Ed's reason is and why the police are not releasing it, but it would be a lot better if Ed and the police would tell us why it is just now being released.
If Moul saw a blue van back on the 8th, and these people saw a blue van why is this fact just coming out now? The public could have been keeping an eye out for the blue van. The police showed other vehicles to obtain information from the public.
If LE believed that Elizabeth was still alive then alerting the public to look for this van could have forced the accomplice to kill her. They were problaby hoping to find him on the quiet in the hopes that they might rescue her.