Oil is sold on the market. I don't see Singapore or Luxemburg sending out warships to get their share of the oil. Nor would we have to, but we want to defend other interests.
Singapore and Luxembourg don't because they depend on us to do it for them. Singapore is a great example actually. I had a friend from there and he described their military as such: they have a small defense force, designed only to hold off any attackers long enough for Uncle Sam to arrive.
Anecdotal evidence aside, under the US-Singapore Strategic Framework Agreement, our navy regularly rotates combat ships through Singapore.
The United States is the largest consumer of oil in the world, double that of the next highest which is China even though we have one-quarter of their population. Relative to other countries, oil has a grossly disproportionate impact on our economy (just ask anyone who lived through the 1973 oil crisis) and so we have no choice but to protect those shipping lanes. Other countries just get to reap the rewards. It is unjust, but there is nothing we can do about until we wean ourselves off of oil. Which is the main reason I think switching to electric cars (even if it means a little government help) is a very good idea.