The obelisk is a symbol of satan. It's always been there since the start of Islam and probably even before. I am not sure if it was Prophet Muhammad, peace be upon him, who originated the obelisk in Mecca. But be as it may, it is the symbol of satan and it is used as such in the Hajj (pilgrimage). This is why Muslims throw rocks at it.
So to place the obelisk on mount Arafat, which is one of Islam's revered mountains and is used in Hajj, is very wrong.
I hope this clarifies things, Insha'Allah.
Take care,
Osama Abdallah