The FAA has sanctioned pilots for landing gear up, as a violation of 91.13 (careless and reckless).
For example, a case from 2006: Administrator vs. Lorenz, NTSB Order No. EA-5205. There was only minor damage to the aircraft, and no damage to anyone else or anyone else's property. The pilot appealed to the NTSB, arguing that since there was no "potential endangerment" there was no violation. The NTSB upheld the FAA's position and the pilot lost.
Here are a few key points (the case has more details and references to other, previous cases):
In particular, respondent, who holds a commercial pilot’s license,
admits that on March 23, 2004, he inadvertently landed a Beechcraft
Model V-35B at the Eastern Iowa Airport with the landing gear
retracted. The aircraft sustained “minor” damage, and no other
property was damaged.
Landing an aircraft with the landing gear retracted clearly creates
the potential for injury, death or significant property damage, not
only to the aircraft and those aboard it, but to those persons or
property that are, or reasonably could have been, in the vicinity of
Interestingly, the NTSB appears to say that who owns the aircraft is irrelevant, because even if it belonged to the pilot there's still "potential" danger to others who might be nearby:
We need not address respondent’s arguments regarding the aircraft’s
corporate ownership and whether it constitutes “property of another”
in light of our finding that his gear-up landing created potential
endangerment to other persons or property in the vicinity of the
Per an AOPA article, it's unusual for the FAA to pursue a 91.13 case for a gear-up landing:
While there are a few of these cases on the books, our experience has
shown that the FAA does not usually choose to take enforcement action
in these situations, choosing instead to handle the matter
administratively with a reexamination request or remedial training in
order to be sure the pilot has the proper procedures in place to
prevent a recurrence.
Perhaps in the case of Lorenz, there was more context or other factors that made the FAA bring the case. I couldn't find any more details about it.