Excellent question! I believe that in the past we have bound a sheet of decals into issues of FineScale, but it was before my time here.
One problem is cost. Model Railroader has a much larger subscriber base than FSM, so they have more discretionary resources (read $$$) for such bonuses.
Another problem, recently discovered, is the post office. The very issue of MR you're talking about led to the PO informing us, I am told, that they require extra postage above the standard periodicals rate in order to include such items. (Either a new regulation or one that nobody knew about before.) Why? Decals are printed on paper just like the rest of the magazine, aren't they? Government agencies, you know.
So it may be quite a while before you see a free decal sheet in MR again, let alone FSM. However, an idea has just occured to me.....
.....How would you feel about the idea of posting a decal sheet here on the website in a PDF file, which could be downloaded and printed out on some of that special inkjet decal paper? (This process is pretty common in the model rocket "cloning" community.....)
Any thoughts?