Garnishment, like a court judgment against you or a lien on your property, is public, and is therefore not regarded as something in which you have a legitimate and defensible privacy interest. Therefore, while it is unprofessional to share knowledge of it with anyone who does not have a legitimate need to know (though note: the company owners do have a legimate right to know, since they ultimately are responsible for what's done with payroll), it is not illegal to do so. So yes, they can in fact do this, unfortunately, even though your concern is certainly understandable.