Obviously, there are some taxis in Aruba that have the light on the roof and others that don't. Natalee's friends were accustomed to seeing taxis that didn't have the light on the roof and thought Natalee was getting into a taxi when she got into the Kalpoe car with Joran.
Obviously someone left their group and wasn't paying attention to their environment and got into a vehicle with someone they only knew for two days tops. Personal accountablility doesn't only belong to bad people, or the OTHER person. jmo