This is how FR can advertise "free" seats - they just add the costs to other parts of the booking - mostly in areas that are "optional" - and so avoiding the EU regulation to ensure the "headline" price includes all additional payments.
By the time you add up the total cost of the booking and include what other "full fare" airlines give (also take into account the cost of getting "door to door" as FR dont fly to the city centre airports) there isnt much difference in total cost.
FR are fine if you live near an out of the way airport and want to go to an out of the way destination, and dont expect any kind of customer service - as with many things in life, you get what you pay for...