Does it really matter? Here's my take.
Laying a wreath in Arlington is a symbolic gesture that is pretty much meaningless. I can hate America with every fiber of my being and still go out there and plunk a wreath down, wave a little flag, and have all the patriots schlobbing my knob. It's a nice thing to do, sure, but it's not the most important thing the president does. It's not even the most important thing he does on that day, probably.
In the past, presidents have realized this and sent delegates when they're not in town. Reagan, Bush Sr, and Bush Jr all did this, and nobody questioned their patriotism.
Furthermore, if McCain were in office and you were his advisor, and he said, "Hey, I've got things I want to do in Arizona this weekend, think anyone would care if I sent Palin instead and laid a wreath in Phoenix somewhere?" You'd be like, "Well, sure, whatever, who cares. The memorial day wreath-laying isn't something I look forward to, and I don't check the TV schedule in the morning so I can be sure to catch it." Because honestly, when was the last time you even paid attention to it?
But for some reason everyone is now a patriotism expert and claims that they've always considered the memorial day wreath laying at Arlington to be super important. And to that, I can't roll my eyes enough.