Matt's answer is right enough, and Axioplases's description does have historical accuracy, but I felt differently enough to propose another answer.
First, here is the truck in question, with the words
カンガルー便 written on the side, "backwards".
Note, though, that the text for the parent company, Seino, is the "right" way round, presumeably because it's in romaji, which is not as flexible as Japanese kanji and kana in terms of direction.
Is Japanese text flexible in terms of direction? A little. The root of the issue lies in the fact that Japanese text is traditionally written vertically. The choice to go left or right when writing horizontally is therefor somewhat arbitrary. Back in the day, one might have come across writing right to left more than today, as in this old train sign:
Again, note the romaji goes left to right.
Why does this historical sign matter? Because it's evidence that there is a cultural basis for accepting text right to left that makes the truck sign possible in Japanese culture. Consider if a delivery company in an English country wrote "Yreviled Ooragnak" on the side of their trucks, because they wanted to follow a "front is top" logic. It just wouldn't fly, because the horizontal direction of text is locked in for our language.
Of course, the massive exposure to English text has influenced textual presentation in Japan, to the point where you don't see this much anymore. But it's not a law that writing must go left to right, and some people will still go right to left, like this racist jerk here, who probably did it precisely because he doesn't want to play by the west's rules:
So the "backward" phenomena is not just about vehicles or flags, or a front-as-top logic. Japanese text, insofar as it has escaped English cultural hegemony, can be flexible about which horizontal direction it goes in, though admitedly it's rare, and probably dying out. (Except with right wing jerks).