I think the assumption is that he has in-demand skills that would be used in a job at home, while the work he would be doing overseas could be done by unskilled labor. Put another way, for a highly skilled technology worker, it is better to help fill a labor shortage in their field of expertise than displace a worker that is in a region with a labor surplus. Even without charitable contributions, the economics work out better for everyone involved.

Admittedly, this assumes both a labor shortage in the technologists field and a labor surplus for unskilled workers in the foreign country, but I think both of those assumptions are supportable.