Light curtain as *start* switch?

I came across this question the other day, and it gave me pause. This fellow has a station set up where the machine automatically starts up when the operator removes his/her hands from the area, as detected by a light curtain. Presumably it stops again when the light curtain is interrupted.

Can you see anything intrinsically wrong with this? I have to admit, it makes me a bit queasy, since I'm used to seeing more explicit start commands (even two-hand controls) required in such instances.

Granted, much may have to do with the particulars of the equipment involved, but are there any generalizable principles involved here?

Ken Crater