Indefinite Articles – Should ‘A’ or ‘An’ Be Used with Abbreviations

It's a and not the because the phrase is part of an attributive clause describing scammers. Here, looking to make a quick buck off the tragic death of a beloved entertainer tells us what type of people the scammers are. It's very likely that, to them, the important bit is that this person is someone famous, not that it's Robin Williams specifically. They would be glad to use this ploy to make a quick buck off the tragic death of any celebrity who had died. Thus, the precise identity of the deceased (which is what using the would mean) is incidental; any beloved entertainer would do, hence a rather than the.

The case of MJ, the pop singer is different because the pop singer is an appositive clause serving to identify a particular, unique Michael Jackson among the various famous ones. If instead pop singer were being used to qualify rather than determine MJ (if he were much less famous and the reader needed some additional context to know who he was, say), then using a would be appropriate, and it would tell the reader a bit more about him. For example, consider this conversation:

Alice: I ran into Michael Jackson yesterday.
Bob: I have no idea who that is.
Alice: He's a pop singer.
Charlie: Who did Alice meet?
Bob: Michael Jackson, a pop singer.

I'd say it is. We still don't care about the book itself, just the act of reading it, so we can keep leaving the book "unspecified". It also reads nicer - "reading a book" is a fairly natural phrase, while "reading the book" brings attention to the book itself.