Unsportsmanlike Conduct or Personal Foul

[Ed Hickland]

This came up at our meeting.

A23 is sprinting downfield toward the goal line for an apparent touchdown.

A73 seeing the opportunity to get back at B74 executes a legal but unnecessary block on B74 at B's 20.

What is the call?

Is it unsportsmanlike conduct penalized from the succeeding spot?

Or, a live ball personal foul penalized from B's 20?