While I cannot see how this would be illegal, my thoughts is that it show that it is possible that Mr. Trump (Sr.) was not being truthful when he says that he knew nothing about Russian interference and that it was fake news. This is not the smoking gun were looking for but more of a spent shell casing.
After months of denial by the Trump team about communication with Russia during the 2016 election, Donald Trump Jr. confirmed he met with a Russian lawyer in hopes of receiving damaging information on Hillary Clinton.
Would such a meeting be illegal? Most experts we talked to said at the very least it raises serious concerns about whether U.S. federal election law was broken. But they also cautioned that the factual record is still developing.