Why did the USA support France in the Second Indo-China war when they were supposedly in favor of decolonization as suggested by point 5 of Wilson's 14 points?
A free, open-minded, and absolutely impartial adjustment of all colonial claims, based upon a strict observance of the principle that in determining all such questions of sovereignty the interests of the populations concerned must have equal weight with the equitable claims of the government whose title is to be determined.
Edit: Title should be "Why did the USA support France in the Second Indo-China War"
The United States was one of the main belligerents in the Second Indo-China War which is usually called the “Vietnam War” in the west. Therefore I assume you are asking about US financial and military support for France during the First Indo-China War (1946-1954). To answer your question, The United States objected to, and initially refused to support the French in Indo-China, until international events caused Vietnam to become a Cold War conflict.
Background
In 1946, following Japanese occupation, the French sought to return Indo-China to its pre-war status as a French Colony against the wishes of the United States. This move was resisted locally by the Viet-Minh: a nationalist resistance movement dominated by Ho Chi Minh and the Vietnamese Communist Party. This initial part of the conflict did not see much foreign intervention on either side.
Start of intervention
Everything changed in 1950 with the success of the Communists in the Chinese Civil War. The Viet Minh began receiving Chinese support in the form of weapons and military advisors and as a result the French began receiving US military aid. The War in Vietnam had moved beyond a purely post colonial conflict and had become, along with the cocurrent war in Korea, another geo-political flashpoint between the forces and proxies of the Soviet Union and the United States. From 1950 on the US began expanding its aid program, eventually including planes, bombs, CIA direct support, and gifting the French an aircraft carrier (USS Belleau Wood). However support was not never-ending or unconditional. Both President Eisenhower and the US congress, (including Lyndon Johnson as the Senate Majority Leader in 1954) were hesitant to commit US troops to the conflict in Vietnam. Reasons for this hesitation did include France’s unwillingness to consider a serious independence program for their Indo-China colony.
To answer your question more broadly, the United States foreign policy during the Cold War era was often pulled in competing directions. In some cases, like French Indo-China, the desire to help post colonial states came in conflict with the stated US policy of resisting the expansion of communism. Cases like these sometimes saw the US act against previously stated interests, as the spread of communism was seen as a greater threat than supporting European colonial projects. Furthermore Wilson’s 14 Points, were very much his own creation and not established US foreign policy dogma. More guidelines than rules, they were only feebly enforced by the US. Which was often more than ready to compromise or delay self determination if they felt the global political situation demanded it.
Let me know if you have any additional questions or need me to cite specifics.