You could also try to set up a proxy in the US, if you do so your steam currency will change to usd.

Is that legal?

pretty sure it is, I didn't get any problems when I did it, nor is it stated anywhere to be illegal.

Just tried and used proxy.
My currency still in Indonesia Rupiah, can't be changed to USD even If I used proxy.

You could try Google Chrome's incognito mode and use Steam through the browser.
Atleast that's a solution that worked for me.

If that doesn't work you can also try this little program.
http://www.hotspotshield.com/
it has a free trial mode you can use with which you can use an IP from the US.
It changed my currency last time I used it.

Tried with Google Chrome's incognito mode.
And, still unable to change my currency to USD.

I'll also try the hospotshield tomorrow.

This is probably a pointless suggestion but have you tried clearing your page data cache in your Google chrome settings i had the exact same issue as you at one point and doing this fixed the issue for me.