And, they get away with it because, for most people, disease, illness, stress, or suffering in reptiles is difficult (if not impossible) to determine. I've seen reptiles on the verge of starvation offered for sale to an unsuspecting public. The death rates of captive reptiles (and amphibians, collectively known as "herptiles" or "herps") are far greater than those of any other commonly-kept "pet" species.
Reptiles seek to keep a specific body temperature for any given period of the day, or a specific activity, which varies according to the individual's specific needs and activity. There are times when they need to be one temperature and times when they need to be another. Thus, they must seek, within a variable environment, to adjust postures and to move from one location to another that provides "microclimate" variations too subtle for us to easily detect. That they can do this owes everything to their having evolved within the ecological conditions where they naturally occur.
Accurately replicating those conditions is required for their overall health and well-being. Such bodily functions as digestion are seriously, and negatively, affected by the wrong temperature - and yet, that cannot be seen by simply looking at the animal.