Does a head of lettuce really need to be refrigerated?
It is all due to storage lifetime.  Lettuce is particularly perishable, compared to many other vegetables.

You will get a considerably longer storage lifetime if it is refrigerated, rather than kept at room temperature.

According to the USDA:


  Storage Conditions: Lettuce should be quickly cooled and
  maintained as close to 0 °C (32 °F)  as possible with 98 to 100% RH.
  Head types are better adapted to prolong storage than are the other
  types,  but none keep longer than 4 weeks, and about half that time at
  5 °C (41 °F). Film liners or individual  polyethylene head wraps are
  desirable for attaining high RH; however they should be perforated or
  be  permeable to maintain a non-injurious atmosphere and to avoid 100%
  RH on removal from storage. Lettuce  is easily damaged by freezing, so
  all parts of the storage room must be kept above the highest freezing
  point  of lettuce of -0.2 °C (31.6 °F).


They follow up later in the document with this chart showing how lettuce's respiration rate increases with temperature&mdash;and of course, once harvested, the lettuce only has so many nutrients available to metabolize:



You can see at warmer temperatures, the lettuce will expire much more quickly.