This is true, in Illinois. However, restaurants can do whatever they like with “service charges”, though customarily those go to the service staff. Automatic gratuity is already illegal in Illinois, or at least illegal to enforce, so a lot of restaurants already apply a service charge on large parties… easy enough, and perfectly legal, to expand that and claim it as revenue.
And that’s just the legal route. Wage theft is rampant in this industry, at least in Chicago.
Source: Am current server in Chicago for over decade.