I find it very very interesting that this (1) went to a GS, and (2) that it covered the summer travel season (its through 7/31). That is both an unusual promo period and an unusual group to sent it to.
OP, have you flown substantially less recently?
My guess is that for this type of a promo, they are seeing fewer high end advance bookings into the summer and a further loss of HVFers, and so are giving away miles to try to gig some further purchases.
Not a good sign if you are of the view that United is doing well and improving its revenue picture.