You think but for vulnerability disclosure, this playing field would be even?

I don't its about things being even.

The USA, its citizens and businesses have more software in business critical and life saving roles than other countries. For example: A 0-day in NGinx is far more likely to be able to harm the US Economy than to help get foreign state secrets. Sharing this exploit or publishing a patch protects US interests from foreign attackers far better than hoping for some foreign state to leak some secret.

Groups representing the US taxpayer should be acting on behalf of the taxpayer.

They are representing the taxpayer. They're doing it in a way you don't approve of, but any of us can say that about any number of things the government does!

I'm pretty skeptical that anything the FBI is doing is altering the balance of power in software security between the US and China.

It's also a little weird to me that we have the expectation that a government agency is going to shoulder responsibility for ensuring the software we run is secure. Shouldn't that be the vendor's responsibility?

Of course it is the Vendor's responsibility. But if the NSA, CIA, FBI, Homeland Security, Secret Service or any other government group gets in the way how much of the blame goes where when the shit hits the fan?

Most vendors will patch if they know about issues. Who is at fault when a hack happens and the the US government could have prevented it by divulging.

None of these groups are "getting in the way". They're doing independent vulnerability research. Their work doesn't prevent you from doing the same research and finding the same bugs. It is really hard to understand the argument that they're somehow doing harm, simply by learning things and then not sharing them with you.