The reason you are being told to use (2) is that it is not an average that is being claimed each year, but rather 1,500 lives that are being claimed each year. To make it clear:
- An average has been claimed each year.
- 1,500 lives have been claimed each year.
To make it clear - because the lives are claimed, you use the plural form of the verb, not the singular.
Firstly, let me say that the data, on the general form a/an NOUN.sg of * NOUN.pl has/had,1 supports my intuition. However, let me exemplify with some examples.
a-1. The colors of the rainbow are beautiful
a-2. The color of the walls is beautiful
b-1. An average of 1,500 lives are claimed each year
b-2. An average of one life is claimed each year
The primary difference between (a) and (b), is that in (a), of acts as as post-modifier - the subject, or perhaps focus of the sentence is the color, not the rainbow or the walls.
In (b), however, of signals a predeterminer - it quantifies the noun phrase itself, and the focus of (b) is life/lives.
There is some level of intuition involved here, but the main test is asking oneself what the focus of the phrase is - what exactly are you talking about? What is most important to the phrase? Unfortunately, there's no procedure for determining which it is in every situation.
1. NOUN.sg = any singular noun, NOUN.pl = any plural noun, * = any word