The future people had the ability to time travel. Once they knew how the virus was initially released, they could have killed/stopped the lab assistant before he did anything.
Instead, they let him get on the plane. Remeber he flew to 7 major cities and released the virus. He'd already opened one of the virus vials and infected people at the security checkpoint. If they wanted to stop him, they wouldn't have let him get that far.
Remember at the end, the lab assistant sat down on the plane next to a woman who was from the future (one of the scientists). She was there to either contract the virus so she could take it back to the future to create a vaccine or she was there to somehow acquire a virus sample from him.
When she was asked by the lab assistant what line of work she was in, she responded, "I'm in insurance". In other words she was insuring that he completed his mission of spreading the virus.