Suppose someone said: "People drive 5-15 miles over the speed limit. It's obvious, then, that speed limits have no effect on how people drive."
It's a pretty silly argument, isn't it? People drive 5-15 miles over the speed limit in large part because they can get away with it. If they started going much faster, though, they would rack up a pile of speeding tickets. And if you raised the speed limit by 10 miles, they'd probably start driving about 10 mph faster.
Again, I'd say this is a pretty silly argument. Politicians deviate moderately from public opinion is that they can get away with it. If they started deviating much more, though, they would be putting their jobs at risk. And if public opinion changed, politicians would probably change their position by about the same amount.
In both cases, the logic is the same. You can usually bend the rules and get away with it, so most people bend the rules. That does not imply, though, that the rules don't matter.