Well, a siege was often used to starve the enemy out, yes.
But the _reason_ you did that was because charging straight into the enemy castle - which is far more fortified and gives the defending troops an advantage - is suicide unless you have a force three times as big as the enemies. Think about it - 100 archers can slaughter 300 soldiers if they have walls to hide behind, and the enemy have no easy way of retaliating other than by shooting back. Which won't work, because the archers have cover. So you tried to starve them out, weaken them by cutting off their supplies, and bombard them to kill enemy troops BEFORE your assault began.
Again, as others have pointed out, dwarves are very self sufficient. So the meaning of siege changes. It's still a siege... only the intent is not to starve. The intent is to batter down the fortifications and then charge the dwarves, who have the numerical disadvantage. (Goblins and humans (I think) breed faster than dwarves, even though dwarves are tougher) I think it's still considered a siege even if the starving out part is not implemented.
"Siege engines" were still called siege engines even if they were used in a flat-out assault rather than the strict definition of "siege", which I do not support, honestly. To me, siege is a term that means to use tactics that even the odds against a force that's dug in and entrenched in their castle/fortress/underground hall, whether by starving them, using machines, or just using sheer superiority of numbers.