The markets are growing as the middle class in these countries expand and demand more and more consumer goods. Trade is the key in exploiting these markets and the American military umbrella is urgently needed to counteract the new Chinese "movements". Even India, which has a huge middle class, eager for "western goods", needs the assurance of the American presence, as it also has territorial disputes with China It is however the recent Chinese claims, that is of immediate importance, especially its declaration of a "fly zone", which was immediately challenged by America, by flying two "unarmed" planes through it. Japan did so too, encouraged by the American flight.
These small disputed "rocks" are important for the wealth in oil, gas, fishery and other resources around them. China with its growing demands for more and more resources to fuel its growth and feed its teeming population is adamant about its claims and without the American presence may try to bully its way over its smaller and weaker neighbors. Thus it was somewhat fortuitous that the recent havoc/ damage to lives and property in the area, especially the Philippines and the very swift and overwhelming American aid response, has opened the door for the USA to return to the Philippines. It used to have two major military bases, the largest outside the USA in the Philippines ( Subic Bay and Clark Air Force Base), which it was forced to leave in the 90's as relationship had soured as a result of the years of support for the deposed dictator, Ferdinand Marcos. Now as a result of America's immediate and abundant response, in the face of the recent disaster, the Philippines want them back and America is eager to do so as if fits well with its Asian Pivot.
The question is now about China's response. Chinese success is tied to its trade, especially with the USA, and with its investment in many countries in, America Europe, Africa and Latin America, Australia and Canada. It cannot lose these. As such its response has to be tempered and this allows Pres. Obama and his Asian Pivot a greater chance of bearing fruit, and helping to restore America's influence in Asia, which was in decline after the Vietnam fiasco.