The C5 and the C4 shut the fuel off at the redline, if they just shut off the ignition, fuel would still be introduced by the injectors. That wouldn't be good.
Now on to redlines...
One of the reasons there is a redline, (not to be confused with a cut-off RPM), is due to valve float at high rpms. At very high rpm the valve springs have difficulty closing the valves quickly enough to keep up with the high RPM. To compensate for this, the design engineers have to plan for higher valve spring rates, but this also creates higher internal friction in the valve train. Overhead cam engines can get away with higher RPM because there are less reciprocating components to contend with. Remember the more components in a given system the more reciprocating weight must be changing direction.
So, in a pushrod design engine, starting at the camshaft, the lifters are moving, the pushrods are moving, the rocker arms are moving and finally the valves are moving. At 6500 RPM the cam is rotating at 3250 RPM, that means each cylinder's valve train (assuming an 8 cylinder engine) is moving at 406.25 cycles/minute in a four cycle engine. This means that each cylinder's intake and exhaust valve is opening and closing at 203.125 cycles per minute, or 3.38 cycles per second. If I did the math correctly.
All those components between the camshaft and the valve are moving at the same rate or cycles / minute. So it's very easy to see why the pushrod design engine does normally have lower RPM limits than it's overhead cam cousin, and why redline limits are so important.
Remember the valvetrain problem is just one stumbling block in the road to engine design, there are many more that affect operating RPM limits also.
vettepilot