I want to offer a slightly different perspective.
Tengoku v. Jigoku isn't about good and evil, it's about order and chaos. Rokugan represents order, a society based upon the precepts of knowing one's place, not about virtue and kindness. Even the Rokugani concept of compassion isn't actually that compassionate so much as it is a matter of avoiding flagrant tyranny or despotism - things that are very good triggers of revolutions. If one's place is not forcefully made more odious, then keeping it is simpler. Blasphemy mostly includes speaking out against The Celestial Order. Desire, Fear, and Regret are just different aspects of refusing to accept fate or circumstance. Ambition isn't a sin because advancement requires the support of one's betters so it drives a person to excel and perform their existing duties with extraordinary skill.
Tengoku and Rokugan are about responsibility, consequence, and fatal acceptance of divine justice. The heavens will see to their own.
Jigoku is injustice, opportunity, freedom and defiance. **** the shackles of society, I deserve more!
With this, the taint requires a contagious nature to fulfill its purpose of being that unseen, unjust, unstoppable horror to which none are immune. If you can't get it unwillingly, then it just isn't a threat.
Shouldn't be then there also a Tengoku taint that makes people less tolerant and less flexible towards ideas that fall outside the box? the Shadowlands taint is not about being evil, why is it then important to be nevertheless seen as threat and thus being contagious? I would say your argument leads exactly to the opposite conclusion, that taint should be not contagious and it should be no threat, since it is not about evil, but about chaos.
There isn't a "Tengoku taint" on Ningen-do because Tengoku is a significantly less Jealous realm. If one were to travel to Tengoku, yes, you would be effected by Tengoku's control, which fills a person with the lawful nature of the realm and moves them closer to the virtues of Bushido. Very little has been said about it because Tengoku doesn't try to expand into other realms like Jigoku.
The reason the Shadowlands Taint exists is because it is Jigoku itself insinuating itself into another realm in an effort to transform that realm into more Jigoku. The reason it focuses so much on Ningen-do is because Ningen-do is spiritually close to many realms, in essence it gives Jigoku a gateway to more realms than any other.