The UK visa application process starts online but involves a personal appointment at a visa application center. Where you can have the personal appointment is determined by your current address, rather than your country of origin.
This can be inferred from the FAQ page of the online visa application system which states:
Can I change my appointment location?
If your appointment is made through Visa4UK and you have also paid online, then you can click the 'View Appointment' followed by the 'Update Appointment ' option in your application. You can use this to change your appointment location within the Visa Issuing Office Region options displayed. If you are looking for an appointment in another region or a different Country you will need to raise a new application with the relevant new location details. If your appointment is made on a Commercial Partner external website, then you will need to contact them.
Thus, as you're currently living in the US, that is were you're expected to apply for a UK visa from.
Also see the UK government's web pages about applying for visas in the US and tourist visas in general.