The probe can't move so that's not a problem. If it gets peed on, the pee is a very thin puddle under the probe and the substrate gets damp. It warms up quickly (it's already at least 80f and possibly warmer). If the snake lays on the probe it actually causes the heat to build up faster causing the thermostat to turn off power to the UTH.
From what I can tell, at some point someone who had some experience with snakes started telling people to put the probe under the terrarium and over the UTH and told people it's the only way to do it safely without actually testing or thinking about how heat travels. Thermal dynamics/ conductivity is something I have studied a lot. It's simply not true that putting the probe inside is as dangerous as some will have you think.