Are the seats marked as completely unavailable or are they marked as requiring an additional charge to select?
The real issue is that in this day and age airlines make available only a very limited number of seats for "free" selection by passengers who don't have FF status and purchase discounted tickets. While overbooking does happen, it rarely results in passengers being involuntarily denied boarding. Every passenger with a confirmed ticket eventually will be assigned a seat; although it may not be together with any traveling companions or in their preferred location (aisle, window, front, back, etc.).
It stinks, but passengers without status may need to determine if they want to pay extra for preferred seating assignments and get seats together or where they want to sit, or if they want to wait for the free assignments and sort it out after that. In some cases the latter works out fine because a gate agent is able to reassign seats before boarding but there's no guarantee.