The legislation some of you are looking for is most likely addressed by FAR/CS 27.561 ( and the equivalent FAR/CS 23 for fixed wing) which effectively states that any item of mass in the cabin which could break free in the event of an accident and cause injury, must be secured to specified G loadings. It really doesn't matter if it's a clamped-on camera or a fire extinguisher, the regulation is the same. In the new CAA guidance document, they have provided a route to ensuring compliance with this requirement for small cameras within prescribed weight limits without the need to have an approved modification in place. However, this only applies to CAA regulated annex 11 aircraft and not EASA types.