someone on TV (sorry I don't remember who) said that anyone who is a suspect or even a person of interest should not ever be involved in a search like this. Supposedly it is legally and ethically improper from both sides of the equation. For example: JVS finds something of Natalee's while searching. He could dispose of or otherwise hide the item if it would deflect attention away from him. On the other hand if JVS found something and divulged it, it could used later to make him appear guilty. Basically, JVS participating in the search in any way is a major conflict of interest and both the Holloways and JVS could potentially be harmed by it. I don't have a source, but it does make sense.
He was the last one to see her. There was no "person of interest" at the time. No one accused of anything.