I think it's about a 50-50 split between LeMond KNOWING Armstrong cheated and LeMond's ego. He was the big-name American cyclist before Armstrong came along (talk about a big fish in a tiny pond); it seems like he sort of misses those days.
Plus Armstrong is a hard dude to like for a lot of people. Yes, he came back from cancer that would have devastated a lot of people, and that's impressive as hell, but on a personal level he's kind of an ass.