How can A rod be rated from 30#-80#. My experience with a rod like this has told me that it will probably be best suited for 40#-50#. Why do they do this?

To provide the broadest possible acceptance by the consumer. It will work reasonably well out to either extreme, but as you noted, such rods tend to be more suited to the middle of the range given.

..........

Also, it seems that if a rod blank has a FAST action-taper, that the manufacturer is inclined to suggest a lower line / lure rating.

For rod blank whose style description & technical specifications do not report a taper-action (nor report a wide lure rating that corroborates the wide line rating), I take this wider range in line rating to imply a softer tipped rod with some lower backbone. At least that seems to be a useful assumption until proven otherwise, based on experience or some other evidence to the contrary.

As Mr. Kirkman has said elsewhere and in times past, there are essentially no uniform industry standards regarding ratings for line, lure, power or taper-action. ... (Hence the efforts of the CCS.) ...

So there is always an arbitrary aspect to rod ratings from the manufacturer, and from the customer, and from one rod-builder to the other. ... C'est la vie. ... And that may never change very much any time soon. -Cliff Hall+++

Well said Cliff!

Raymond Adams
Eventually, all things merge, and a river runs through it..