Thank you all for your answers, it helped me a lot.
Indeed, the LA Building Import @kartonage mentioned (https://wiki.openstreetmap.org/wiki/Los_Angeles,_California/Buildings_Import) says that 17 936 "“Warehousing, Distribution, Storage” buildings were imported as “building=warehouse”, including 8 396 under 10 000 square feet.
After an aerial imagery verification, it seems that a lot of shops having a storage space are tagged as warehouses.
I can’t find a way to select and remove this kind of buildings (very few tags building:use=warehouse).
How to map and code the complexity of reality, such a hard question !