In this case, the indefinite article is optional. It is OK to use it, but not necessary. The omission of articles is very idiomatic. There is not a simple rule to memorize that works all the time. Here is a good chart about using articles. http://depts.gallaudet.edu/englishworks
I see it as a subtle difference, comparable to "that" versus "which."
"I like the painting that is hanging on the wall" implies that there is only one painting on the wall, whereas "I like the painting, which is on the wall" implies that there may be several paintings on the wall. Similarly, "Many in Taiwan don't consider the island to be a part of China" implies that it is (or isn't) on