Mostly they're conspiracy theorists.
See also water powered cars etc. etc.
Normally they claim that mega-corporations destroy or sit on the technology because otherwise we'd have free energy and they'd be out of a job.
Usually conspiracy theorists are low achievers who get to feel important by convincing themselves that they have knowledge that others don't or are not smart enough to accept.
By waging a publicity war on these supposed enemies they get to feel important and powerful.
Of course if it (or even something like it) were real someone would be making billions through patenting and licensing the technology
You'd buy your Zero-point-energy car and contract to pay a monthly running charge or something of the sort
The technology changes but the businesses stay the same.