Where VSL are concerned I think on the whole they're a good idea and if used/controlled properly can help smooth the flow or traffic and/or increase safety.
However, unless there's a warning to the effect of "speed cameras in permenant use" having the camera working when there isn't a VSL in place is a little underhanded.
Now I know I'm probably going to get preached to about exceeding the speed limit being a cardinal sin and that anyone who does so needs to have their right foot chopped off to make them think twice about speeding again but.........
If there's a speed limit, you make a conscious decision as to whether you're going to exceed it or not. That decision may be influenced by a number of things, for example if you think it's safe to do so and the likelyhood of being caught is low then you may be more inclined to take your chances and travel a little faster. I've heard of numerous examples where in good concidtions, police patrols have turned a blind eye to cars travelling up to 85mph.
My understanding is that the idea of speed cameras is to improve road safety.... well that's what the police and other authorities keep telling us so let's take that as gospel.
If when entering a VSL zone there was a sign warning that speed cameras are in constant use, it's likely that the vast majority of drivers would obey the 70MPH limit, thus the number of people exceeding the speed limit would be less and the road would be safer?
Otherwise not warning that the camera are in constant operation *could* be seen as an underhanded tactic in lulling drivers into a false sense of security to generate revenue.
I guess it comes down to individual authorities/organisation who run them to decide how/when they operate rather than a blanket rule?