I'm trying to find a reference I read not long ago along these lines.
If I'm not way out of line and totally mistaken, the point is making the other steel and the weld zone come as close as possible to matching the corrosion resistance of the Stainless Steel. And it is rather difficult to only galv the one part. Thus the entire finished product must be galvanized.
Seems like there are other processes that would apply as well.
Have a Great Day, Brent