I suspect a fair portion of the seat reservation no shows are business travellers.
The online booking system my employer users requires you to select both outbound and return journeys. Only after you select these journeys are you given the option of choosing the type of ticket you want, with an open ticket being one of the options. After you select the ticket the next stage of the process is seat reservations, by default the system reserves seats on the previously selected journeys, you have to manually de-select them if you don't want reservations.