It is quite simple.
Canister shot will not fire on it intended target unless one change to free fire.
The Sherman HE will fire on intended target unless one set the weapon to fire on vehicles.
Doesn't make sense. Your argument is effectively: "well the Sherman comes with vehicle prioritization already turned off so it's fine" as if the USF player can't just turn on vehicle prioritization after the fact.
Clearly, if we extrapolate your argument here, the solution is to have M-42 enter the field with vehicle prioritization turned off. That way, M-42 AI shot will "fire on intended target unless one set the weapon to fire on vehicles".
If that doesn't make sense to you, that's because it doesn't make sense.
The fact is there is no huge difference between the two. If you're advocating for this change to the M-42, you might as well advocate for it to be done to the Sherman as well.