Good question VM44.
The reason WT makes the announcement in the KM encouraging all young JW men of age to register with the Selective Service System goes back to 1918 when J.F. Rutherford and seven other WT leaders were indicted and taken in custody on federal charges of conspiring to violate the Espionage Act of June 15, 1917. The allegations behind the federal charges were that the WT leaders were conspiring to obstruct the recruiting and enlisting of men in in the U.S. Armed Forces during a time of war. The WT leaders were convicted and each sentenced to about 80 years imprisonment. They appealed and the U.S. Court of Appeals in New York later ruled that the WT leaders did not receive an "impartial trial" and a new trial was ordered which never happened. The WT portrays this as evidence that the WT leaders were not guilty, but the U.S. attorney for the case, L.W. Ross, officially stated in writing that he did not want to pursue the new trial because of the possiblity of it causing the U.S. government negative relations with the general public, not because any of lack of evidence. All of this can be read in documents recorded with the U.S. National Archives.
Ever since those federal indictments and convictions, WT leaders to this day are very careful to cover their ass when it comes to their published view of registering with the U.S. Selective Service System. The WT encourages young men to register, but if those same young men are imprisoned for refusal of duty in the armed forces, the WT will imply the young men made that decision to refuse duty based on their own conscience not because the WT directly instructed them to refuse, which could bring another federal indictment upon the WT, especially with the news laws in a time of war or terrorism.