Because they have solid grounds, thanks to Obama himself, to assume so..
A recent Gallup poll found that one in five Americans believe that President Barack Hussein Obama is a Muslim. Reporting on the poll, VoaNews opined: "The number of Americans with that mistaken idea has risen in the past year, although President Obama is a practicing Christian and has made many public statements about his faith."
While such a large number of Americans could be mistaken as VoaNews asserts, but their assumption is, at all, not without foundation. On several occasions, Obama has admitted that he is a Muslim. There a number of Youtube videos, showing Obama confess that he is a Muslim.[1] In a private meeting on Saturday, June 12, 2010, Obama told Egyptian Foreign Minister Ahmed Aboul Gheit: "…he was still a Muslim, the son of a Muslim father, the stepson of Muslim stepfather, that his half-brothers in Kenya are all Muslims, and that he was sympathetic towards the Muslim agenda."
Don't these confessions in his very own words give one a solid ground to assume that President Barack Hussain Obama could, indeed, be a Muslim?
Apart from this, Obama attended a racist anti-White church for 20 years (for whatever reason), but since he cut ties with his former racist pastor and that church, after latter's virulent attack on the Whites during the election campaign, he is yet to find another church to pray (is he bothered to find one at all?).
From another angle, since Obama’s father was a Kenyan Muslim, he is by-default a Muslim for being born to a Muslim father. And he has not openly said that he renounced Islam. Had he done so, he would be an apostate of Islam in the Muslim eye, and his life would be at stake. Instead, when he toured Egypt, Saudi Arabia, Iraq and other Muslim countries of Middle East, he was welcomed and greeted everywhere as an honoured guest and a friend of Muslims. Can an openly 'Muslim apostate' expect such warm welcome from Muslim leaders and peoples of the Middle East? While he could be spared, given his position as the president of America, he could never ever expect the warmest welcome reserved for a foreign leader from the fanatical Arab Muslim populace and leaders.
Moreover, Obama's confession as a Muslim to the Egyptian Foreign Minister is much more revealing:
…he asked that the Moslem world show patience and promised that once he could overcome some domestic American problems, he would show the Moslem world what he would do with Israel.
The Israel-Palestine issue is closest grievance to Muslims all over the world, so, it seems, is for Obama.