There's at least three ways around that. In addition to being able to be locked out both electronically and mechanically, many lights have a switch that is fairly well protected. I may not trust my TS10 rattling around loose/unclipped except in my otherwise-empty watch pocket, but hitting the button on most of my Hanklights is such a low-probability thing that the only light I lockout is my KR4.

It's also worth noting that I have the memory set to "hybrid", so unless I used the light at a high level within the last three minutes, it'll come on quite low in the highly unlikely event that something manages to poke inside the raised ring anyways.