Because Samuels never affected events on the field? Sport dies if it isn't perceived to be a true spectacle. Look at the tour de france, it's impossible to win it without being questioned now. Imagine how Stuart Broad felt when he found out the best innings of his life was devalued by the knowledge that the Pakistani bowlers had better things to do than concentrate on him.
Should cheats and fixers be banned for life? Yes, no doubt for me. Amir wasn't, so he'll come back, and he'll be the subject of much speculation for the rest of his career. Hopefully he doesn't make the same mistake again or cricket will look foolish.
As he is coming back, hopefully the ICC will be wise enough to use him for anti-corruption causes, similar to David Millar in cycling. Millar was accepted back after doping because of the outspoken anti-doping campaigner he became, unlike others who carried on denying it and stayed under doubt for the rest of their careers.