Lots of theories out there as to why studios release early internationally, nothing definitive. The reasoning that makes the most sense to me is that collectively, international accounts for about 2/3 of most blockbuster theatrical and is growing much faster than domestic, and studios always want to open in the biggest market first. Domestic market on the other hand is mature and not growing, and they know they'll make the same $$$ whether they release it here first or behind international (sucks for us fans but it's probably true). What's counter to this reasoning though is that to call international a single market is ridiculous, it's incredibly fragmented and requires coordination with a wide number of distributors by territory. However, I'm sure the major studios all have some quantitative research to back up this release strategy.
As to why not a WW day and date release, I suppose it's because local distributors want to be able to pick out weekends for these tentpole films to maximize revenue. Release dates differ by territory and you don't want Avengers going up against Batman on the same weekend in China, for example.
