I am currently experiencing this frustration! I can do StW but only to PremiumPlus. If they can show me StW then surely UA could show buy up prices when PlusPoints are applied.
With StW I am more likely to use a combo of StW for some flights and buy up for other flights. Instead if I have PlusPoints applied I have to wait until OLCI for the buy up. Missed money making opportunity for United to get my money earlier.