Excellent point. They don't charge YQ, but their taxes and fees are minimally higher on UA award flights than SQ would charge. SQ Award fees are in $SIN, but are the exact same fees as UA charges, i.e. $5.60 per leg domestically (except from Canada to US, which is more). AE charges just over $12 CAD for the same leg. A round trip booked b/w US and Canada on United with SQ miles would be the equivalent in $SIN of approx. $69 USD. With AE, it's over $120 CAD. Not a lot more, but a bit.

I agree with you, though, unless you're booking a lot of these flights, the minor differences in taxes and fees isn't worth transferring Starpoints to SQ for United flights. Much better bets would be AS and AA for US carriers.