I suppose it would be where they draw the line. Is me "paying" for the flight and my friend paying for the accommodation costs while on holiday a purchase agreement?
Unless USDM could categorically prove something along the lines of those companies that sell award seats online, there's probably minimal risk to helping out a family member or friend. If not then it gets down to questioning why did you gift miles or if you share to someone and get nothing in return, is that from the goodness of your hear or a financial transaction?
The brand damage of leaving someone stranded over a false claim for breaking their T&Cs is probably not worth it to them. I'm not sure how they work out if you've bought a flight from one of those online award flight sellers. Maybe too many bookings from the one IP?
Considering that I specified to USDM I wanted to book a flight for my friend so he could travel to ICN with me, or when my brother made a booking for me, if they had an issue with that then it should occur before parting with the points + cash.