[Walt Longmire]

Why would they do this?

Because he is a free agent at the end of the season and they hadn't been able to sign him to an extension. They got 2 top 100 prospects in the trade as well. But the bug thing is they were able to get the Dodgers to take David Price's horrible contract. They had exceeded the luxury tax threshold too many times and they were getting ready to suffer some big penalties concerning international bonus money and draft picks. By getting rid of Betts and Price's contract they will get under the luxury tax threshold and reset all those penalties while kickstarting the rebuild of their farm system. It wouldn't shock me if they try to resign Betts after the season.