i personally think this policy is despicable and stupid too. it puts united on a par with southwest and jet blue and jet blue is often cheaper.
It will cost united some customers that way, but it will really, really tick off a whole bunch of united customers and cost far more that way
it also is probably illegal to give customers less than a year's notice that they will lose their miles. If i remember right this is less notice than other airlines gave and probably would be viewed by state attorney generals as insufficient.