None of the Saleae devices are electrically isolated. That means that the ground pins on the Saleae device are connected to the USB ground. When you connect the logic analyzer to the device, you will effectively connect the device's ground to the PC's ground. If your device and PC are powered from AC main power, they will probably already be common grounded. In this scenario, if the ground pin from the logic analyzer is connected to any voltage other than ground on your device, ground current will occur and can potentially damage your device under test, the Logic analyzer, or the host PC, or harm the user.