The way the tanks are manufactured, they put water in them to get the excess metal shavings out of them, and they don't dry them properly, therefore they get some surface rust.
What you can do to get rid of the rust inside the tank is to fill 1 cup of gas only, no oil. Fill the tank, making sure to plug the bottom hole and put the gas lid back on properly. Then shake the tank for a few minutes, this will get rid of the surface rust inside the gas tank.