You've got your facts wrong. Your 750 watt PSU is using a single rail design. That's the problem here. That's why the cables melted. If the PSU had properly configured multiple rails it would not fry.
Now, the 1250 watt version uses a multi-rail design but has the OCP limit for some of it's connectors set too high (45A). Setting the limit to 45 amperes can also result in fire if the connection is not tight enough or the cable is very long or even very thin.
Proper multi-rail design with OCP set at sane levels: System shuts down instead of catching fire when treated with too much current.
Improper multi-rail design: Either the system shuts down with no reason, or cables melt when it's treated with too much current.
Single rail design: Cables melt when the system is treated with too much current.