Residency checked and enforced at SEA?
 

I use http://www.skyscanner.net/ to find the best deal, which directs me to book a one-way rental from SEA at http://www.affordablecarhire.com/. After making full payment and receiving the rental voucher and confirmation number for budget, I pull up the reservation on www.budget.com which has the following:

under personal info: residency GB
under required credential: If the country of residence on the renter's credentials do not match the country indicated on the reservation, the rental rate will change.

I hold a Singapore driver's license and will be visiting the US.

Shall I worry?

I certainly would be concerned. Time to contact affordablecarhire and get them to fix it.