Is there any reason a PFC would wear the trouser stripe with their ASU?

As far as I know, the military farms out much of its PR and advertisement stuff to civilian contracted agencies. If I had to guess, this was a photo of models in uniform and they decided for some reason that they would all look better in the same pants.

I can't imagine if real soldiers were used that a PFC would shell out the extra for the stripe that he could not wear during normal wear of the uniform.

There are reasons, none of which are supported by DA PAM 670-1 (e.g., ate up)

The closets legitimate reason could be that it's an old picture, when the highlighted paragraph was authorized.

Yes. If the PFC is a member of an Honor Guard of some sort then they would wear the trouser stripe so that they are uniform with the NCOs that also perform in the Honor Guard. An easy example of this is the 3d Infantry Regiment, or Old Guard. All of the members of the army drill team will wear the stripes. In addition, Tomb Sentinals will do the same. However, I'm pretty sure that Sentinels still wear the old style dress blues in which the stripe was worn regardless of rank. Only recently with the adoption of the ASU did we turn to a Marine Corps like tradition of granting that decoration to NCOs and higher.