You're not seeing it because you're thinking in terms of internal US economics, but this is a question about international economics and geopolitical leverage in the distant future.

The reason to consider the ban is because it might be the only way to preserve a fully autonomous and independent American frontier AI stack and the long-term strategic value of possessing such a stack could vastly outweigh the cost of giving up true free market competition on AI. If giving US startups and other companies access to cheaper Chinese AI means sacrificing the US's ability to own its own frontier AI stack, is that a rational trade, or would it severely and irrecoverably sacrifice the country's technological autonomy and leverage for decades to come in exchange for cheaper tokens for a little bit early on?

If the US not only gives up most of its manufacturing capability to China, but also allows itself to give up its own AI stack and become almost entirely dependent on foreign AI, then it's conceivable the combination of the two sacrifices will deal a permanent deathblow to the country in exchange for what will turn out to have been a couple decades of cheap goods and AI tokens.