Just like using the Tridon CM gui can't copy content across publications, you can't copy content using a core service function.
You can off course do the following
- read the component you wish to copy from publication A
- create a new component in publication B
- copy all data from the original component into the newly created component.
You will have to pay attention with file names. If publication B is a child of publication A (or the other way around), you will get an error if you try to create a component with the same name in the same folder. If both publications are not in the same Blueprint chain than naming conflicts may be less likely but are something your architecture must account for.
A question similar to yours has already been answered, you can find some useful code samples here