The reason no president has met with NK before is that it legitimises the regime. It gives NK the thing that they want the most - the prestige and propaganda of sitting down with the POTUS. By meeting KJU, Trump is giving him equal status as a state leader. When Clinton was president he was smart enough to send an envoy to find out what deal was on the table before deciding to decline NK's invitation.
Just look at how the White House is trying to walk back on what Trump said. The press briefing yesterday added all sorts of caveats to a meeting taking place, which essentially adds up to what every administration before has said. Trump, as usual, jumped the gun and shot his mouth off after NK offered a meeting. Does the US even have an ambassador on the Korean peninsula right now?!
Obama, who was generally the most open president towards meeting old foes said of NK: "Since I came into office, the one thing I was clear about was, we're not going to reward this kind of provocative behaviour. You don't get to bang your spoon on the table and somehow you get your way."
I know most people won't remember this, but Obama was slated by both Republicans and Democrats when he suggested that he would be in favor of meeting with the renegade leaders without preconditions.