Korea - Hotel Service Charges in Korea

Can someone please clarify if all major four/five star hotels in Seoul include a service charge? I recently stayed at the InterContinental and it specifically had a 10% service charge broken out in the billing, but at the Park Hyatt only the room charge appeared on the billing. Am I to assume some hotels include this as part of their rate, others add it on to a base rate? I noticed that foreigners do not pay the sales tax on hotel stays.

Slightly OT:

I noticed that foreigners do not pay the sales tax on hotel stays.

I've read the same thing somewhere. If I try to make a reservation for the JW Marriott, the taxes are applied automatically. As the rate I'm looking for is a "advance purchase", ie a prepaid rate; how would I get these taxes back?

I did a prepaid at the Hyatt ICN and my confirmation noted "tax excluded". The actual billing was for the base amount without any tax applied so even if your Marriott booking page shows a base rate plus tax, you should only be billed that base rate as long as your card and address are from outside Korea.