Lying about it during a confirmation hearing? Now that's illegal.
At this rate they'll introduce a resolution soon enough that if you have eaten pierogi in the past 10 years you can't hold a public office.
There are plenty of reasons why one would not disclose a meeting on a public record, from even forgetting it happened to the fact that having it in the open might jeopardise negotiations/operations/assets etc.
It's much easier to get things done in many cases when they are shrouded by secrecy while people might not like it but if we need something from the Russians or want to send them a message sometimes it's best not to be done as an official state visit and vise versa.
Every negotiation at some point started as a non-official meeting in the kitchen of some restaurant somewhere or a hotel lobby.
> The Washington Post contacted all 26 members of the 2016 Senate Armed Services Committee to see whether any lawmakers besides Sessions met with Kislyak in 2016. Of the 20 lawmakers who responded, every senator, including Chairman John McCain (R-Ariz.), said they did not meet with the Russian ambassador last year. The other lawmakers on the panel did not respond as of Wednesday evening.