Rust (ferric oxide) is formed from Iron (the FE in steel) and oxygen, with water as a catalyst.
Deprive the FE of oxygen and you prevent rust.
Used to be, just keeping the tank full of gas would do that. As the humidity (water) in the air, along with the oxygen, couldn't reach the iron. No rust.
Alcohol changes things considerably. Alcohol absorbs water (from the humidity) and that water contains dissolved oxygen. Putting this (contaminated) fuel next to iron or aluminum creates corrosion from oxidation. Worse, the presence of water and oxygen in alcohol begin to form alkali which actively attacks metals by chemical conversion. It is as aggressive as acid for destruction of metal.
Alcohol fuels must be used and renewed constantly, or as MRieck notes, your fuel system will begin to destruct.
Just another benefit of lobby groups in a short sighted government.
Cheers,