At the grocery store here in Germany I saw that lettuce isn't refrigerated. So I brought a head home and it's been in my pantry for 3 days and it seems fine... The English speaking internet says that's not ok, I don't know what the German one says.
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:
They follow up later in the document with this chart showing how lettuce's respiration rate increases with temperature—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.
Usually sellers hold the lettuce at room temperature during the hours of sale (to show to buyers) and place them in the refrigerator in the night.
But attention. The supermarkets assemble the heads under plastic films (as said), which retain moisture, and daily keep them anyway in departments "fresh." The small vendors display the salads without protection, but often spray water, to maintain the freshness of the leaves, which otherwise would wilt.
The result is the same of cut flowers placed in a jar with water.