This why the oaths are different...

If this is the case, however, then why is the distinction made between the two oaths when both enlisted and officers are not obligated to follow unlawful orders according to the UCMJ?

Officers, especially at higher ranks, have a unique position of authority and influence within the organization that could be taken advantage of for political gain. Swearing loyalty to the Constitution instead of the president or any other person means that officials cannot manipulate officers in order to gain control over the military and become dictators.

http://www.quantico.marines.mil/News/News-Article-Display/Article/611510/the-difference-between-oath-of-office-oath-of-enlistment/