My answer depends on whether, in your example, one really hears the 'R'; I speak British english, where - except for the west country accents - we don't rhotacise vowels. I'm not sure how we "justify" the pronunciation of 'a' as 'er', but I can tell you what it is so that you can find more information. If the 'r' is only slight, then the phenomenon is called a 'schwa' and is a weak vowel, in an unstressed position. It is actually far more common than one would thing, and can involve all the other vowels; examples include: - the 'a' in about; - the 'e' in taken; - the 'i' in pencil; - the 'o' in eloquent; - the 'u' in supply; and - the 'y' in sibyl [ˈsɪbəl]. Of course, in some American accents the schwa is further rhotacised (given an 'R' sound) and then you hear Obama pronounced (exaggerated) 'ObameRR'. Here is the Wikipedia entry for more information: http://en.wikipedia.org/wiki/Schwa

Some Americans do it too. It is a form of hypercorrectionism found primarily in non-rhotic accents (accents where they only pronounce R's if there are vowels after them). Because they do not pronounce the R's in words like 'water' or 'actor' except if the next word starts with a vowel, they end up pronouncing them 'wata' and 'acta'. Thus, to their ears, there is no reason to distinguish between the last vowels in 'wata' and 'Obama'. When the word 'Obama' is followed by a vowel, they say 'Obamer. Anyway, you do not 'justify' the way you pronounce stuff, especially not with a language with an orthography as irregular as English. Accents change very drastically with time, and spelling is almost always slow to catch up. Hope this helps!

There is no need to justify it. That is what we do. In fact, hundreds of syllables are pronounced just like "er".