The taxes on sugar were to prop up the (heavily subsidized) US corn industry. Why do you think they even water down the gasoline with it? There's so much of it, and it's so artificially cheap, and they want to keep demand high to keep the farmers happy.
The taxes on sugar have nothing to do with corn. Corn producers do not need HFCS to stay in business, they have ethanol when they need a subsidy; less than 5% of US production goes to HFCS, but the rest is basically split 50/50 between animal feed and ethanol.
Sugar imports are taxed so that US producers (cane in FL & LA, sugar beets in the upper midwest and upper great plains states) do not have to compete with cane sugar from Brazil and Mexico. It is as simple as that.
