Obama doens't believe them to be adhering to Islamic teaching. Those members of ISIS do believe their actions are justified under their religious views.
To say that Obama is justified and accurate in declaring them something other than the religion they claim to be is to state that every fundamentality is justified in doing the same type of thing.
ISIS claims to be Islamic. They seek to instill Islamic tennents in government. They identify as muslim. Obama has no standing to simply tell the American public "they're not Islamic". Yes. Yes they are. They may be a sect of Islam with beliefs and interpritations of their beliefs that you disagree with, but they are none the less Islamic.