The GPS device was attached to his vehicle, which is driven on public, state and federally owned road infrastructure. There is no legally defensible expectation of privacy in public places. His car is registered to him, with a license plate that ties him to the vehicle. Tracking him visually by having agents follow him, or tracking him by GPS signal, is nominally different both effectively and physically. There was no breach of privacy, there was no attempt to prosecute this man for anything. The FBI has the constitutional right to track him in public places. The individual also has the constitutional right to avoid being tracked, as this individual did by removing the GPS tracker.
Now, if they wiretapped his telephones and recorded all of his conversations without a warrant, that might be a little different ... but that's what the Patriot Act specifically allows.