The difference is in enforceability.
Open weight models are less like Napster and more like DeCSS -- once you have the digital artifact, there's little external evidence you're using them.
Napster was easy to target because it was an open P2P network and specific key US individuals.
If the US government banned open weight models tomorrow (national security grounds), they'd already get a lot of pushback, only increasing day by day as more 'less than SOTA' solutions using them are deployed.
The US government could likely enforce this on its own supply chain (military and federal contracts, maybe some state funding) easily enough.
Enforcing it on private companies would be more difficult... maybe they could push that through, but it would likely take Congress to pass a law. And Congress is substantially less enamored with supporting OpenAI / Anthropic / Google / Meta.
And even if that gets pushed through, enforcement is going to be a bitch on smaller companies using non-US clouds.