People tend to have a completely inaccurate view on this because of bad popsci, out-of-context feasibility studies from the 1950s and fiction that just leans toward the "fantasy" on that.
The reasons for this is are very simple:
1) There is no "high ground". Being in orbit does not enable you to throw things "down" for free, thats simply not how orbital dynamics work. All the impact energy you have to pay for up front, and delta-v on some orbital object is almost inevitably more expensive than the equivalent amount of explosives on the ground.
2) Anything launched from beyond earth orbit would be unusably delayed. And telegraphed.
3) The concept does not have a single selling point from a military perspective; not really faster than an ICBM, penetration is limited by newtonian physics regardless of speed and more expensive than just delivering the equivalent amount of explosives (or nuclear payload) via missile.
edit: Yes an interplanetary transport system could be weaponized. But so could the explosives currently used in mining (~tens of millions of tons per year), no one cares.
And we're not even talking about "orbital" bombardment. We're talking about a continuous stream of material at well above escape velocity, whose delta-V is already paid for. Simply redirecting a few pieces of this stream would not be more telegraphed than any other attack. By the way, are you sure the control systems of the mine owners are resilient to coercion of their executives by "legitimate" terrestrial governments? I'm not. None of your reasons are effective.
The energies are substantially more than mining explosives, with wildly different properties as far as threat detection and mitigation. To put it charitably, that's a quite silly comparison.
Edit: now that I'm thinking about it, maybe the best mitigatiom would be to extrude all the mined material into some other very high surface area shape that can't survive reentry, no matter the mass, but can survive being accelerated for the trip home. Maybe a big long truss, with the engine at one end. And then also limit the payload size.