This language appears in the KrisFlyer terms and conditions:
"For industry discount, agency discount or award tickets, infant fares and other special discounted tickets as specified by Singapore Airlines or airline partners from time to time, no KrisFlyer miles will be earned."
I'm sure you can find similar language in the MileagePlus terms and conditions. Or perhaps the fare basis of your KrisFlyer award ticket will be one of the non-mileage-earning fare bases if you look at the MileagePlus earnings chart for SQ flights.