It's still a pagan idol --- should he kiss a statue of Buddha or bow down to a Hindu god or article of worship? Or to show respect to New-Age pagans should he kiss a crystal?
Did the pope say he was worshiping the Koran? For all we know he was praying that all Muslims would be converted to Roman Catholicism. You'd have to ask him what he meant by his gesture. To you it means one thing; to him it probably meant something very different. You can bet he wasn't praying to it.