The police aren't interested in investigating low-value cases. However, with a suspect in sight they'll go after them even if it's low-value.
To answer my own questions, police do not care so much about the "value" of the target. Their behavior is much easier to explain once you realize that their job is to protect capital.
Also, the victims of wage theft are less likely to be "well connected" sponsors of political careers, while the victims of larceny are naturally more often business owners whom are more likely to have a "voice".
Furthermore, cops aren't exactly skilled at digging out such things. Wage theft is something for the department of labor, not the cops.