If you come to the U.S. on a visa that’s explicitly labeled a “nonimmigrant” visa for people who are “coming temporarily to the United States to perform services,” then it’s not “cruel” to actually enforce that. Those words are literally in the law.
The law is ever changing and is not always a reflection of what's right, moral, ethical, etc.
You have plenty of historical examples of this, most prominently slavery being legal.
It's ok to defend a thing, but just because the law says so is very rarely a good argument.
The law doesn't describe reality, though. The so-called "non-immigrant" visas are really not that. "Non-immigrant" has a specific legal meaning, and like many legal terms, they don't match up with what you might consider everyday usage of the term.
And even if they were truly non-immigrant, who cares? If someone comes to the US, does good, useful work, and stays out of trouble, I want them to be given the opportunity to stay permanently. You may not, perhaps, but, well... I don't care.