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The UK allows applications from anywhere, providing the applicant is legally in that particular country. There is no requirement to apply from country of residence.
To apply for a UK visa in the USA you need to:
apply online
pay your fee
book and attend your biometric appointment
Attend an Application Support Centre (ASC)
post your application
Source: https://www.gov.uk/government/publications/usa-apply-for-a-uk-visa/apply-for-a-uk-visa-in-the-usa