While I agree with you, that world-wide is what counts; it seems that for a lot of people, it's really only domestic that counts.
Even the Hollywood trade mags/sites have articles on how a movie is doing using only the domestic figure. I find that frustrating when I'm looking for the worldwide figures... but that's the situation.
And again, while I look toward worldwide figures, it should be noted that international markets can skew comparisons.
- If a film maker 'localizes' the movie with dubs in many languages, they do better than studios that don't.
- Also, American movies have themes, scenes, and types of people that some international markets ban, which makes international comparisons skew a bit.
- Also, the release schedule overseas can be completely different than the domestic release date. Thus, the final take of a movie sometimes isn't know until months later.
- And movies that show up later in the worldwide schedule wind up doing less than expected since the movie had been pirated and 'distributed' before it hits foreign theaters.
So... domestic figures for domestic films are more reliable for direct comparisons since in the U.S., there is little language barriers and no government censorship.