If it is not giving up the draft choice, I can understand that, maybe. But, signing Snell also would give the Angels a starting pitching surplus, if there is any such thing.
One SP to trade, with a couple of teams woefully short in SP, like the Yankees and Red Sox appear to be, offers an opportunity to replenish the farm system after last summer, when we dealt two or three of our top prospects away in deals that obviously didn't work.