Nope, Montag has it right. Really bad officiating. As a fan, I wanted to see what was going to happen in the rest of the match. Instead, a poorly trained or supervised line judge decided the match.
I’ll ask you like I asked all the others: How do know it wasn’t a foot fault? Were you sitting on the baseline watching Williams’ feet?
No. It was her second serve having missed her first serve and she couldn't win second serves against her opponent. Most likely, she would have lost the point. She denied herself the opportunity to play against a 15-40 lead by her tantrum.