Civilians of many other cities were targeted during WWII as well, so it was certainly standard practice.  That so soon thereafter, the civilized world collectively declared targeting civilians a war crime by the 1949 Geneva Conventions suggests to me the would-have-been offending governments themselves simply operated without regard to any moral compass.

So, to me, classifying it simply as 'legal' (until 1949) would be a bit more accurate than 'just'.