They didn't, no, and had no explanation why it was so done (just acknowledge that "it sometimes happens, we don't know why" -- as throughout the online process it just showed it as $ - only when I checked my DL receipt and my card statement I saw that it was Singapore $ and not US $, and yes, it was accordingly higher as per the conversion rate). I had to open a dispute with AmEx and have them credit back their 3% foreign currency penalty here, after explaining that I did not authorize Delta to charge in a non-USD currency and that the fact that they would be doing so was never disclosed to me.

The issue with a non-USD transaction is, unless you used a CapitalOne or a higher end Chase card or such which has no foreign currency conversion penalty, you will be hit with a (likely) 3% penalty for the non-USD charge on top of the conversion rate.