it's not pure copper lines - that would be too soft. this stuff is cunifer aka copper nickel lines, that can handle higher pressures of hydraulic systems. it's the stuff most european cars use - porsche, volvo, audi, etc, some racers know about it, and it's DOT approved. big advantage is that it's easier to bend than conventional steel line.
and according to the fedhill site, theoretical burst pressure for 3/16 brake line is around 17,000 psi.