What makes you think the same equipment would have the necessary mechanics and capacity for realtime mass-filtering?
Even in America, I imagine there are many other budget priorities competing for a limited spook-fund, that would displace a "national censor-wall but indefinitely inactive and secret just in case we need it someday."
You know how the internet and things like Tor are products of the Department of Defense?
The State Department has been using the internet (and anonymization tools like Tor) to organize dissidents in foreign countries for various purposes, often coups. One example is the Arab Spring[1].
I'd wager they are very afraid of someone doing it back to them, and might have some capability in place. Even if it's just to shut the internet off for a time (like
Pakistan and other nations have done during elections[0], maybe in response to US interference) or perhaps prevent connections to other nations/the rest of the world more generally.
So the issue isn't a PRC-esque Great Firewall, but instead a more generic "blanket interruption of service in some region for some time"? I feel those are significantly different scenarios.
For one, they're would be an enormous backlash if it were to somehow effect the many American businesses which rely on network access and have significant clout in our system. Most of the countries that have tried such things either (A) don't have the economic exposure or (B) limited the outage-scope to zones without the same stakeholders.
Another aspect is that such coarse interruptions are a lot easier to accomplish through a bunch of NSL-weilding lawyer-agents contacting ISPs, rather than spending money building dedicated hardware infrastructure, in advance, in secret, to support a Giant Red Button.
I'm not saying there's no possible Motive, but it's no substitute for Means and Opportunity.
[0]https://en.wikipedia.org/wiki/Black_room
[1]https://en.wikipedia.org/wiki/Room_641A