This was just a case of the FA making a call on her own with no policy to back it up. I am not usually a fan of Southwest Airlines but I do like how they manage drink service on many of the shorter routes. As soon as you are seated and before take off the FA goes around taking drink orders. They don't take out the cart and the FA just comes around with a tray of drinks and passes them out.
US based airlines are capable of full catering even on the shorter routes. I used to take the DL Shuttle from LGA-DCA regularly back in the day and because it was a highly profitable route you got a free beer or wine and a snack bag. If you ordered a soda they gave you the whole can and a cup of ice so they didn't have to slow down to pour.