You'd be correct if aerospace companies from other countries had as much of a right to rent the pad as SpaceX, but that's not the case.
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.
There is no underlying platonic reality here to be discovered around the concept of a subsidy. It's not entirely unreasonable to be careful about allowing foreign governments to launch large rockets on domestic soil.
The entire booster stage of the Antares rocket, which is only used for NASA payloads, comes from Ukraine. And Antares and Atlas V's main engines come from Russia.
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.
Rockets launched from the US are already considered to be malicious from a range safety perspective, since they might accidentally veer off course and turn into a ballistic missile.
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.
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.