Is it normal for adverts for US jobs to give much less information than European ones?
In Europe, at least in science, a postdoc is generally hired with specific funding to undertake a specific project. This might not be the case in the US, where lab heads will take on postdocs with a general idea of the area they will work in, but then it will be up to the postdoc to design their own research program. 

Also note that the law around employment in the US is much less restrictive, so where in Europe an employee can call foul if asked to do something not in their job description, and an employer can only sack an employee if they prove to be below a what would reasonably be expected on a listed responsibility on their job description, these things don't apply in the US.