Oh, I see what you mean now. Sorry for the confusion.
The key difference between those examples is Steele wasn’t representing a foreign government. If the Trump campaign hired investigators (American or foreign) to look into Hillary Clinton or her associates, that would be normal opposition research. Every campaign does that — I’m sure Trump’s did too — and it’s both legal and legitimate.
Natalia Veselnitskaya was introduced to Donald Trump Jr. as a representative of a foreign government, acting as part of that government’s efforts to influence the election. That crosses a line, and might be illegal.
One reason the U.S. draws the line there is because something like Veselnitskaya’s proposed meeting could be a foreign intelligence operation; a deliberate attempt to get senior campaign officials involved with something of questionable legality, which the foreign operative can then threaten to expose.
As is typical of this sort of thing, the line is somewhat blurry. But hiring Steele is on one side and holding a secret meeting with Veselnitskaya after conducting that email exchange is on the other.
An interesting case to explore is DNC operative Alexandra Chalupa’s research into Paul Manafort with help from Ukraine. It didn’t involve senior Clinton campaign officials, much of the information was published and publicly available, and Ukrainian involvement in the election was much less extensive than Russia’s. But there was still some cooperation between an opposition researcher and a foreign government.
That case is not as clear cut of a line crossing as Don Jr.’s. But it’s closer to the blurry line than Steele’s. It highlights how America hasn’t clearly delineated the difference between legitimate and illegitimate foreign interactions for political campaigns.