In a situation like that, I wouldn't blame him. Consider while building his milling machine how many of these situations he will come across. If he had to make sure there was a hardware failswitch, it would simply not scale.
3D printers are like this too. They have mechanical limit switches [0] that are read only by software. So if there is a bug in the software, nothing is stopping it from pushing the hardware limits and breaking. Same goes the other way around, if this switch is broken, same might happen.
Most 3D printers don't have massive printing heads. If they drive into the end-stops, the motors will likely just skip steps and be stuck. They are not designed to apply much force.
I'm much more worried about the heating element. Its temperature is usually controlled by the same cpu that also does motion control and g-code parsing. If anything locks up the CPU the heat might not be turned off in time, and (because you also want fast startup) there is enough power available to melt something. At the very least you would get nasty fumes from over-heated plastics, and maybe even teflon tape, which often is part of the print head. At worst it could start a fire.
As a 3D printing enthusiast, I can confirm your fears. It's all in software and while there are good control systems, nothings perfect. I had the hotbed fail and it was smoking when I found it.
Note that (depending on how the 3d printer is wired) a defective switch will result in an "endstop hit" condition. A dislodged switch however will happily keep reporting it's not being hit.
3D printers are like this too. They have mechanical limit switches [0] that are read only by software. So if there is a bug in the software, nothing is stopping it from pushing the hardware limits and breaking. Same goes the other way around, if this switch is broken, same might happen.
[0] https://i.ebayimg.com/images/g/EYAAAOSwbopZguz4/s-l300.jpg