It may be a mistake to wade into this, but I always thought the reason the Tstat probe shouldn't be placed inside the enclosure was because the snake may inadvertently move the probe off the UTH. Since the UTH only heats the surface above it and does nothing for ambient, if the probe were moved to the cool side, that could and would cause a dangerous spike in temps.
Other wise SquirmyPug is correct about the transfer of heat. If the snake is laying on the probe then its body is the same temp as the substrate, and probe, and won't cause a spike in temp. If the snake peed on the probe then the animals urine is still the same temp as the animals body, which again is the same temp as the substrate and probe. So again, no temp spike. Of course there is the potential for the liquid to cause damage to the probe.
Not trying to stir anything up or garner any ill will. Just trying to point out that maybe everyone is correct!
