As I suspected, the USBC responded that it is only supposed to be foreign materials placed on the ball by the bowler, so belt marks do not make a ball illegal. However, belt marks and the like are still going to be disruptive to clean when a dry towel is not sufficient and it needs to be addressed by an officer.
Here are two other additional interesting pieces of information from the response I got.
"Patting the ball with rosin
USBC recently changed our interpretation of the no foreign substance on the outer surface of the bowling ball is allowed and based on the interpretation, patting the holes of a bowling ball allows rosin to be added to the surface which is a violation of the rule. Wiping it off with a cloth does not make the ball in compliance with the rule.
Therefore, if a bowler pats the rosin bag onto the ball and wipes
the outer surface clean, the ball(s) would have to be removed from competition. However, since this is a newer change, the bowler may not have been aware and should be issued a warning and the equipment removed from competition. Failure to remove the ball(s) from competition could result in forfeiture of games."
"Removing residue INSIDE a hole
Removing tape residue from inside a hole would be allowed during USBC certified competition with consent from the league/tournament official."
The rosin part doesn't apply to me, but it implies that rosin whether cleaned with a dry or wet cloth is a surface change.
The tape one makes me feel even more sorry for league officers.