The US with ITAR and other "national security" restrictions sets up a lot of economic moats around their aerospace companies, and the government gets to pick domestic winners over foreign competition. That's a subsidy.
Everyone plays this game when it comes to aerospace, it's why there's constant lawsuits between the likes of Boeing and Airbus on at the WTO.
ITAR works in the other direction, when you take US tech into other countries.
Liability for the launching company, now that's a separate issue from ITAR.
Range safety officers already strap explosives to SpaceX's and other rockets and will detonate them in the event of an accident. This is how the SRBs on the Challenger space shuttle were detonated after the orbiter broke up.
So it's not because of range safety that foreign companies aren't allowed equal access to the US launch market. We could go into why that is. Briefly it's because rocketry is a core strategic military asset from both a proliferation and manufacturing perspective. Once you can launch a satellite into orbit you're really close to having an ICBM.
But that doesn't change the fact that this strategic advantage is partly maintained with subsidies to domestic companies, in a way that say doesn't happen when the US military needs to source some more basic technology (like nails, screws or lumber) from some outside provider.
No, it's called renting. If it was unused then the presumably low price SpaceX pays for it is the right one. This is actually efficient capitalist allocation of resources. If SpaceX was not allowed to pay a rent for the launch pad, capital would be used inefficiently on building one, instead of being spent on R&D. Furthermore, the depreciation and abandonment of the launch pad itself would result in a waste of tax-payer money.