Read that sentence again. Like a lot of government legalese, it seems confusing, but what it is saying, is that you may operate it IF it doesn't cause harmful interference.
It's not just a bunch of super-ego pilots getting their rocks off by telling people they can't do things. ANYTHING that puts out RF (radio frequency) has the possibility of causing interference with other electronics. And in some planes, especially older ones with possibly worn insulation on their wiring, you might (probably not, but might) cause interference. There have been verified instances of problems reported by pilots, including myself, caused by radio interference. (We had to sit and listen to a lady talk on her cell phone in mid flight on a Saab 340 - it was bleeding into our headsets. We had to send the FA back to tell her to shut up).
And yes, camcorders DO emit RF from the amplifier for the viewscreen and audio. Like Sleekjet said, just because you got away with it once doesn't mean it won't screw up the system next time. I know it's sometimes hard to sit and take orders you think are bogus, but please, they are for yours, mine, and everyone's safety.
One smooth landing is skill. Two in a row is luck. Three in a row and someone is lying.