Probably because of the 2001 B-52 friendly fire incident when the JTAC's GPS reset causing them to read their own coordinates instead of the target for line 6. We always teach crew to plot target and friendlies IOT check that the line 8 distance and bearing actually makes sense, as well as sensor correlate with the JTAC when possible (i.e, that grid falls at the east side of a bridge over a N-S river allowing a final crew-JTAC crosscheck that the coordinates are where he intended them).
You are correct, ideally there would be no friendly grids on the radio but without TRPs or sensor/visual talk on to friendlies they need some way to crosscheck line 8 in a BOC situation.
No they don't. There is no requirement anywhere in 3-09.3 that says the aircrew must crosscheck line 8 in a BOC type scenario. Sure, given a situation where friendly positions are passed and a 9-line is subsequently passed, they should make every effort to verify the target it not coincidental with a friendly position. This is obvious. In a TIC where you check on and get an immediate BOC 9-line w/o the JTAC asking for your check-in is not the time to play that game.
But you implying that is is a requirement is exactly the kind of lack of CAS understanding I'm referring to that is rampant in the USAF. It's the same kind of thing that leads to stupid SPINS that require read backs of nearly the entire 9-line. It's CYA idiocy.
Your "preaching" of individual TTPs seems to be overriding a thorough understanding of Joint CAS procedures.
To continue my anecdote:
I asked the a/c "What? you want a friendly grid after you read back a BOC 9-line? Not happening. It's in Line 8." After a little more back and forth (and no friendly pos passed), they finally ran the attack and hit the target. Extremely slow execution from read backs to TOT though.