I always thought those springs were a kind of cushion between the cable pull in either direction and the carbs to make sure the carbs don't see too much force. A shock absorber so to speak.
Technically, they are a precision-control method for the dual-cable setup which makes it possible to set up a zero-lash precision throttle. Like so many things CB750, it was the first of its kind, and Honda was concerned that if a [clumsy] mechanic over-tightened the cables in order to provide 'precise speed control' of an already somewhat-jerky throttle that this would lead to premature cable wear/failure and a bad image for the bike. And, like many of these things in the 750, only the 750 ended up having them (industry-wide) because it was, in a word, overkill. Admittedly, the 750 does have a sudden throttle below 25% or so, a situation which was overly remedied in the (much more expensive and complex) PD carbs by using variable bellcrank leverage angles - which became the infamous 'flat spot' in the middle of the 500/550 throttle instead. This morphed into the PD carbs in the last 2 years of the 750, too, but there it did make things smoother when riding in city traffic. While it was done in those later bikes for emissions reasons, it 'cost' other things in those engines and frames to make up for the expense of those carbs - things like cheaper swingarm bushings (and their lube system), cheaper engine cases that needed spacers in the tranny to fit things right, and cheaper valve guides, with simple reinforcement ribs on the cylinder fins instead of carefully-cast struts and airflow ports that required hand-cleaning after casting. Even the headlight fork ears in the later bikes were shrunk to save a few Yen (for the metal's cost), and other things like that. Heck, the 1-piece output shaft to the countersprocket lost the following parts in those bikes: 1.) oil adjuster screw, 2.) oil pan, 3.) oil adjuster screen, 4.) oil metering seal, 5.) oil metering seal mount, 6. Outer mounting nut for all this, with tapped hole, 7.) machined thru-port, 8.) floating sprocket retainer and 9.) its locktab: all those later bikes got was 1.) a tapped hole partway thru the shaft with 2.) a bolt and 3.) washer on it, and an O-ring chain as the 'apology' for it (IMHO)!
