you're making a comparison with different levels of offence I think here.
Chandimal's appeal seems to be based on the fact that the ICC cannot determine what he actually did, but also on the fact that his action was not pre-meditated.
The case against the Australians is clear; three players colluded to deliberately set out to cheat, before play, and used sandpaper to alter the condition of the ball.
Whilst I'm not privy to the musings of the ICC or individual boards, it seems that the ICC have banned Chandimal, but it was Cricket Australia who banned Warner, Smith and Bancroft for the year.
There's a very good piece here on ESPNCricinfo which deals with it in pretty comprehensive terms.
I do see where you're coming from but I think the issue of pre-meditation is critical here.
RE: Dinesh Chandimal gets only one match ban