Why are airports in Greece blurred and low res in Google Maps?
The short answer seems to be that some countries have persuaded Google to blur out specific areas for "national security" reasons. Wikipedia has an article on it with some more (limited!) information. Interestingly, I couldn't find any official Google article on blurring in Maps, apart from some general references to Street View. And, as it turns out, Street View at ATH works as normal.

FWIW, it seems that Greece didn't persuade Microsoft to play along, because Bing Maps has much better images. That strongly suggests that the blurring is mostly security theater.

(By the way, I'm not saying that there are no security/privacy risks with Google Maps, Bing or other tools; it's just that some risks are more credible than others. security.SE would be a good place to ask more about that.)