That is because you don't need them. The floating head switch and the plasma posts (like for Sheetcam) are setup to use the floating head switch on a reference move and to use Z Home input. There also may be problems with noise being transmitted via the "probe" wires into the THC300 box on some machines since the signal has to pass through the THC Sensor card wit all the plasma stuff on it. Next generation of cards won't even have the holes for probe connections. On most machines it was easier to connect the Z home wires back to the Z home inputs on the port 1 breakout than to run them over to the plasma unit and back up into the THC300 and port 2. If you really want to use those pins contact Campbell Designs and I am sure they will swap out the card.
Support for THC300 and MP1000-THC Digital THC are at: