I've always thought it should be "he is the President of the United States."
But I have seen several places where it is written "Donald Trump is P(p)resident of the United States." E.g. here and here. I thought proper noun or not, the definite article should precede the title, since there is only one office in the context. Why can the definite article be dropped?
Similarly, I have also seen sentences like "She was treasurer of the club." For instance Merriam Webster has an example "She is treasurer of the college." But examples with the definite article also abound. Like "The EVPT is the treasurer of the MIT Corporation" and "Swan is the Treasurer of a resource-rich country." Why can the definite article be dropped? If the reason is a plurality of treasures, shouldn't it be "a treasurer"? Which is the common/technically correct way?