The military wanted to know what would happen to a bunch of ships near ground zero of a fission bomb. Apparently they thought it was OK to irradiate a bunch of nearby islanders.
[youtube=http://www.youtube.com/watch?v=-l6Q8Q1smwg]
http://en.wikipedia.org/wiki/File:Bravo_Fallout.jpg
The Castle Bravo shot (which is what the fallout map is from) and the Crossroads Baker shot (the test with the ships) were different tests.
Crossroads (1946) used smaller devices (23 kT) and the area was evacuated except for US military personnel, who were exposed to excess radiation because wasn’t a clear understanding of the radiation hazards at that time. The Crossroads tests became the first real lesson in those hazards.
The Castle Bravo shot (1954) caused a great deal of radiation exposure, primarily by accident. The original shot size was supposed to be around 6 MT, however the designers has missed a reaction and the actual size was around 15 MT. The result was much greater fallout than expected, far beyond the cleared zone.
In neither case did the military think it “OK to irradiate a bunch of nearby islanders”. I’ll never defend the stupid things that they DID deliberately do to the islanders, but if you’re going to criticize them, do it for something they did do (eg. the test exposure of troops during the Plumbbob Smokey shot), rather than what they didn’t.