copy and pasted from your earlier post:
680.74 Bonding.
All metal piping systems and all grounded metal parts in contact with the circulating water shall be bonded together using a solid copper bonding jumper, insulated, covered, or bare, not smaller than 8 AWG.
It doesn't say "any function in circulating the water". An all metal drain piping system certainly has "contact with the circulating water", as any water in the tub would be included.
In addition, have you ever watched a tub drain?? The water does circulate...:thumbsup:
to the great amusement of my 3 year old!!