Right. By law everyone onboard must have a seat and be seated.
Went like this: I went to the Frontier website and put in my origin airport and destination. The page pops up and says "we have low fares to Phoenix for $65.00 *one way." Great! I book two round trip tickets for two people. $260.00 bucks. Then it asks me about checked bags. $80 bucks, something like that. Great! Roundtrip for $340. I'm all set. Nooo. I click "accept" or something and then this page pops up. "Now pick your seat". There is a map of the seating on the plane with a $21.00 price on each seat. So apparently, the $340.00 was just for the airplane and bags to get to Phoenix.
If I want to ride on the airplane it's $21.00 each person, each way.