As the poster above me said, you learn your limits. It is much easier to cut 10-40lbs over the course of a month/few weeks than it is to maintain a very strict diet year around. Also, with the training these guys go through to prepare for a fight they would overtrain very quickly if they weren't eating right most of camp. (During a cut one certainly does not eat right)
I heartily accept the motto, "That government is best which governs least"; and I should like to see it acted up to more rapidly and systematically.