There's nothing that says the seller's agent has to be present at inspections, but many agents (myself included) think it's a good idea.
The listing agent's job is to represent the seller in the transaction. The home inspection is a critical part of the process. By attending the inspection the listing agent can observe first hand any issues that the inspector raises and can point them out later to the seller. If anything is damaged during the inspection process, the agent will be able to explain later to the seller what happened and the seller can then decide if they wish to pursue the buyer for money to repair the damages.
If you are a seller, this is a topic to take up with your agent before the inspections, as not all agents do attend inspections if they represent the seller's side.