Without doubt, the answer is yes, we can give the greeting to nonmuslims. Here are 2 examples from the Quran
To Abrahams Kafir father:
Abraham said: "Peace be on thee("salamun alayka"): I will pray to my Lord for thy forgiveness: for He is to me Most Gracious.
And when they hear vain talk, they turn away therefrom and say: "To us our deeds, and to you yours; peace be to you("salamun alaykum"): we seek not the ignorant."
Besides the obvious Quranic references, because there is no prohibition on doing so, one cannot term the action haram.