The trouble is that thieves are often one step ahead of honest folk. It would be trivially easy to install a tag in most instruments. But for it to be effective, the reading technology has to be widely available and cheap (as it is). This means that thieves would have access to it, and professionals would check if the instrument has an ID. Having ascertained this, they could then easily locate it and remove it. Drunks and druggies wouldn't check, but would be disposing of the instrument back- street at a low price anyway.