Insanely, the Chicago Convention of 1944 exempts international flights from any fuel tax. https://www.icao.int/publications/pages/doc7300.aspx

Various attempts to work round this without rewriting the convention have been discussed, e.g. https://commonslibrary.parliament.uk/research-briefings/sn00... but I don't think they've got anywhere.

The US _might_ be able to tax fuel on domestic flights, but that would result in the California to Seattle flight going via Canada to avoid the tax.