I agree with the GP that the opportunity cost argument is so wholly dependent on the judgment of the debated activity that it doesn't really pertain one way or the other.
That said, HFT doesn't strike me as something that adds value to the economy.
I'm saying it depends on what A is criticizing about B. It is not sinister for A to criticize B's action. I'm not sure if it's sinister for A to criticize B's misallocation of time spent. It's a weird thing to think about. I think it's weird to think about because, as was my point earlier, the judgment of whether the time was misspent is wholly dependent on the judgment of the way the time was spent.
A criticising B is not a problem.
A preventing B from doing something is sinister (life, liberty, pursuit of happiness).