I always thought that the "string" in string beans referred to their shape.  They are also sometimes called snap beans, because you can "snap" the ends off to prepare them for cooking.

The Wikipedia article says that it refers to the "string" along the seam on one side of the bean, but, as I said, I observe that much more often in snow peas.

Oh, well, they've probably all been cross-bred to oblivion anyway.