Ashton may be a bit lucky to have gotten off as lightly as he did, though it is probably due in large part to the fact that Smith was not injured on the play.
Considering the distance between Smith and the boards and the fact that Ashton hit him squarely in the back, this could have been a much more dangerous situation. As Brendan Shanahan explains in the video, the onus is on Ashton to either avoid the check or minimize the contact. He did neither in this case.
With two players leaving games on stretchers after hits from behind this year, this infraction may not hold up against those that led to five-game suspensions for Blues forward Maxim Lapierre and Avalanche forward Cody McLeod, but it's still a bad hit.
The fact that Ashton has no prior disciplinary history also played a role in the length of his suspension.
You hate to see hits like this in the game, but Shanahan and company have not been shy about sitting players down for blatant hits from behind and/or boarding infractions. This probably could have been a longer suspension, but a multiple-game ban is something at least.