They don't. The range of a fully loaded A320 is 3100 miles, 3300 miles if it's a newer plane with sharklets (JetBlue has both). It's 2600 miles in a straight line from BOS-LGB. 2800 miles in reality when taking into account air traffic routing. JetBlue's A320s are probably rarely fully loaded as they only seat 150 people (for now).