Late-night breakfast is pretty common in some parts of the US, particularly among college students and travelers. Hence the not uncommon occurrence of 24 hour restaurants, both chain and local, which frequently serve breakfast all day. (Waffle House, Denny's, many diners, casinos etc. Not that they were going to a casino in SLC!)
Additionally Kenzie had been in California for a few days, an hour behind SLC time, and thus may not have physically felt it was that late. 2 am to college students often isn't considered late, especially in the summer.
Another poster already noted that the flight from LA is not a long one, so while she was undoubtedly emotionally tired from grieving, I doubt she felt much fatigue simply from being on a plane for two hours or so. For all we know she napped on the flight or had refreshments on board, and wanted to wind down a little before going home to sleeping roommates. MOO