I think its quite simply a case of unneccesary frustration from the typically competent driver. If a driver tries to be completely law-abiding and travel at a lower than considered maximum safe speed he/she tends to either lose alertness and concentration or simply focus much more on the speedometer even without cameras (although of course cameras do mostly operate on the roads with bad speed limits). So because at least the first scenario is rather undesirable drivers will just exceed the limit. And while your at it why not try and double your concentration whilst your at it - ie. look out for talivans instead of looking at your speedo all the time.
In 95-99% of cases, speeding is simply caused by badly set speed limits. And allowing speed limits to be lowered because of non-safety reasons (eg. noise) only makes the situation even worse. And of course I share the view like many others that badly set speed limits only create more speeders in areas where speed limits are properly set.
On a final note I thought I might also add that when approaching a speed limit change its often easy and tempting to predetermine whether the approaching speed limit is justified of not by looking at the quality of the signs. So when your, say on a main single carriageway in a suburban area, and you see big, new, sparkling

signs its easy to immediately disregard it as badly set and speed anyway. Whereas if your going through an urban area with

signs which are old, rusted, bent, beaten up etc. its likely to be better observed as its assumed to have been imposed properly before the mid-90s.