Then you run the risk of a bottom feeding team starting crap with a contending team. Then what? You can't defend yourself for fear of suspension and no replacement player?
How much of that actually happens? I'm sure it has, but to that extent, I can't recall it happening too often, but that's why there's an appeal process. If that threat really exists, then create some sort of way to deal with that, too, but bean ball wars are more dangerous than they're worth and it's an issue that baseball wants to take care of (and I'm not sure that they do), then make the penalty strong enough to deter the action.