Not some odd reason - he's an ex-pat in the finance industry. It's like how I had modified my pronunciation of 'cache' (although I refuse to change how I say 'data').
BankSimple are based in Portland, Oregon (so yes, US).
BankSimple are based in Portland, Oregon (so yes, US).
So, it'd be "BankSimple is based in...", instead of "BankSimple are based in...".
When I first started reading things from the UK, I initially thought the writers simply used poor grammar :)