After two years, in fact, that extra 48GB of storage (on top of the 2GB offered new users, for a total of 50GB) disappears unless an end user enrolls in Dropbox's Pro or Teams service. Those start at $9.99 a month for the former and, "it varies" for the latter.
Dropbox's web page describing the promotion is somewhat confusing, because apparently some Samsung devices come with a one-year promotion, although the GS3 comes with a two-year promotion.
Once the two-year promotional period ends, your Dropbox service will automatically downgrade to the amount of free storage space then being offered to all users with free accounts with a bonus 3 GB of storage space. One year promotional accounts lose that 48GB after one year, and get to keep the 3GB of bonus storage for only one year beyond that.
Dropbox didn't go into details as to why those two U.S. carriers decided to skip the promo, but we'd guess it has something to do with the fact that they are the top two U.S. carriers and have been feeling strains on their networks.
In terms of U.S. users being shorted on processor power, earlier, we noted that the U.S. versions of the GS3 will not be carrying Samsung's home-grown 1.4 Ghz Exynos quad-core processor, but instead a 1.5Ghz dual-core Qualcomm Snapdragon S4 processor.
While the U.S. versions will have 2GB of RAM vs. 1GB of RAM, testing using a VFUK version of the GS3 with the quad-core SoC showed it to be significantly faster in the CPU portion of the Quadrant benchmark than the U.S. version.
It's unclear why Samsung has switched to the Snapdragon S4 in the U.S., but it seems like it's the U.S.' LTE networks, but the Tegra 3, a similar quad-core processor shipping in the HTC One X, was swapped out of U.S. versions as the Tegra 3 does not support LTE.
Coincidentally, the Tegra 3 was replaced with the same Qualcomm Snapdragon S4 processor in the GS3.