Why exactly was the line item veto unconstitutional specifically under the presentment clause?

Congress passes a bill. The president gets to veto it or not veto it. He doesn't get to cherry pick the parts he likes. The bills congress send to the president the result of compromises. If the president can cherry pick what he likes in the bill and toss out what he doesn't like the process of negotiation in congress becomes much less useful. The line item veto is a bad idea. It gives the president way too much power. Congress controls the purse strings. The president has veto power. A veto is all or nothing. This is what the founders intended.