I just found this info:

The minimum distance between a wall and the edge of a free standing or wall-hung lavatory is 4". (IRC R 307.2)

A pedestal or wall mounted lavatory must meet both of the building code requirements. The center of the lavatory must be at least 15" away from any side wall or tall obstacle, such as a tall cabinet, and the edge of the lavatory must be 4" from the wall or tall obstacle. There are practical reasons for the 4" requirement. Any closer and it would be very difficult to clean the lavatory.

That is the dumbest thing I ever heard. If it is against the side wall with a tiled side splash why do they still require the 4 inches.