I have an alternative view. First, "пиндос" is not so rude as "faggot" etc. It's still a disparaging word, but no more. Second, I'm sure you know about the word "америкос" (= American, a bit offensive too) and about the word "пингвин" (= penguin). The word "пиндос" sounds for Russians like "пингвин + америкос". You know that penguins are fat, stupid and move awkwardly - like American soldiers look with their ammunition. That's the reason why the Russians soldiers called Americans "пиндосы". It has nothing to do with the Greek word, of course. Just sort of "phonetic transfer" (I'm not sure it is a correct term or not).