15 YARD FACE MASK FOUL - Enforced As A "tack-on"?

[RoyGardner]

In the Colts-Chargers game today a receiver was fouled right as he came off the LOS (a 15 yd facemask). Later in the same play, he caught a forward pass well downfield, ran for an additional yard or two, and was tackled. After several different discussions between the R and the calling official (BJ I believe), and then between the R and the U, the penalty was enforced as a 15 yard tack-on from the end of the run (actually was a 1/2 distance enforcement). Maybe the fact that the flag was thrown by a deep official, and ended up in the area where the runner was tackled caused some confusion?

Was this enforcement correct? I thought for a 15 yard FM foul to be a tack-on it had to be against the runner? In this case the fouled player was not the runner at the time of the foul. What is the correct enforcement for this under NFL rules?

I believe this is the play (from NFL.com):

4-5-SD 34 (3:02)
18-P.Manning pass short middle to 44-D.Clark to SD 18 for 16 yards (32-E.Weddle).
PENALTY on SD-95-S.Phillips, Face Mask (15 Yards), 9 yards, enforced at SD 18.