It's not about whether you mind or not.

It's about whether Y person will get justice.

Let me put it differently:

Police actions need to be transparent. There are strong reasons to release recordings with police actions in them.

But if the recordings are unrelated to any police officers or police actions, then all of that goes away. It's completely fine for police to not release footage of two random people interacting. And that scenario is what is analogous to BART surveillance videos.

  two random people interacting

That's an odd way to paraphrase the pairing of a felony perpetrator and a felony victim.

It's in the context of the previous comments, "X person robbing Y person". I didn't want to be redundant. No matter what crimes are happening, there are no police involved, so there is no police transparency motive to release the video.

The motive is to provide the public with information. Some of this information may be useful to identify the (gang of) attackers.

That's a perfectly fine motive, but it's not enough to force anyone that has such a video to release it completely publicly. And BART in this case is basically an "anyone". They have no particular involvement.