because they are uneducated, and selling your twins in the same manner that they sell recreational mom/pop skis.
twins are softer, center set, and have less running area due to the second tip. therefore, its a definate plus to upsize your skis vs downsize. additionally, MOST kids with twins are pretty agressive skiers, and are able to handle a longer ski without negative side effects. longer skis are also noticably more stable and much faster due to the larger surface area.