You should drain the bowls and disconnect the fuel line before storage!
The picture looks to me like someone left the petcock on and put the bike aside for years. The fuel evaporates in the bowl... but it stays full until the tank empties, concentrating the additives and impurities plus any water dissolved in the fuel (there's always some) is trapped at the bottom unable to evaporate once it drops out of solution.
I don't believe a single bowl full of fuel would do this much damage, that leaves a yellow coating of crud on the bottom of the bowl with some corrosion and a bit of white crust - not total devastation.
Storing a fuel tank is another story - leave it as full as you can (with non-ethanol gas if you can find it) for winter storage: drain, air out, and then coat the inside with oil (2-stroke oil, WD-40, or motor oil) for long-term storage. Putting napthalene mothballs in a sock tied with a ribbon put in the (empty) tank with the ribbon out the cap so you can retrieve the sock (a thin flat ribbon won't damage the cap gasket like a string will) will prevent oxidation nicely - the napthalene vapour pushes out the air fairly quickly and the balls last for a long time.
