The cyclist is indeed correct in that no rear light is required, and the cop is wrong. However, for safety's sake, it's an incredibly smart idea to run rear red lights at night, along with reflectors. And with this guy's rear rack, I can see how the reflector is being obscured. He really should put a light on the rear of that bike.
I don't think his rights were being violated, it was a valid stop if the cop was having a hard time seeing the cyclist. But yes, it could have been a pretense for a warrant check, since most cops check everyone they deal with for warrants just as a matter of procedure. Source: My 15 years as a police dispatcher.
So all in all, this is much ado about nothing. Maybe the cop didn't know the law (and should look it up if he's not sure) but he apparently just wanted to warn the guy that he's not very visible to motorists and send him on his way.