Yes, you are missing something. And those devices are or are not contemporary versions of the perpetual motion machine. That depends on whom you ask.
The original idea of Nicola Tesla was to harvest energy from neutrinos and other stellar radiation, which is freely available. That might be feasible and it would not violate any laws of physics. This idea met esoteric and pseudoscience. This is where the idea took a questionable turn. The members of the community are very persuasive and do not balk from calling engineers ignorant (or something less polite, which I wont repeat on a family friendly website) for predicting the behavior that can be experienced when building and operating those devices. Of course there are plenty of people who embrace the opportunity to make the money that´s in the business of selling plans for building the devices.
My policy is to steer clear of those folks. You cannot win and you should not risk getting sucked into that movement - you would not believe the amount of support and resources my colleague managed to solicit for his projects.