Congresscritters aren't bound by the Logan Act. They can meet with foreign leaders, even overseas, as part of their 'authority" to "investigate" for "legislative" reasons.
Of course, that's not why the Democrats do it...because Democratics think Congress is an advocacy group, not a legislative body.
(And another reminder: Pelosi met with Assad in 2007 as Speaker of the House, specifically to undermine Bush's foreign policy. Just because she has broad latitude for this, doesn't mean she's not abusing it. And she's not the only one to abuse it - Obama's 2008 Presidential campaign include a swing through Western Europe meeting with world leaders.)