Update: at Hot Air, of all places, they have a serious take on the stupidity of this:
This is a recurring problem for Trump, seen most recently in what he said about “Second Amendment people”: He doesn’t seem capable of imagining how the things he says will be understood beyond his own fan base. Tom Joscelyn noted this morning that “Obama founded ISIS” is also an idea pushed by Iran’s supreme leader, Khamenei. He has a different meaning of “founded” than Trump does: He wants Shiites, who loathe ISIS, to believe that the organization was deliberately created and equipped by the U.S. to persecute them. That’s not what Trump means by it, but because he insists on using a word that implies intent in describing Obama’s role, Iran can use the clip of Trump in its English-language propaganda. Trump either doesn’t grasp that or doesn’t care enough to be more precise with his criticism.Update 2: and then comes the argument, from Slate, that Trump knows what he's doing. Personally, I think that's giving Trump too much credit. I think it's more likely that he wings it in front of supporters, because he likes the roar of the crowd and is actually insecure, and then comes the retro-justification:
Hewitt then countered one last time by suggesting that he personally would use “different language” to communicate the same criticism. Trump’s response was remarkable for its awareness. “But they wouldn’t talk about your language,” he told Hewitt, “and they do talk about my language, right?”
That remark is telling, and it illustrates something that should be obvious by now but is often lost in the noise of each new controversy that comes every time Trump says something outlandish and/or obviously untrue. This was not some ad-libbed comment that went awry, a bad joke that did not land, or the candidate going “off message,” as Beltway pundits call it. In fact, he’s completely on message, and this has been the message for years, dating back to Obama’s first term, during which Trump used the birther movement to lay the foundation for his current presidential run. More than anything, Trump has built his campaign on (white) America’s fears of the other, and what better way for him to harness those than by othering the sitting president of the United States, be it by questioning his citizenship, his faith, or his loyalty. It doesn’t matter to Trump whether his wild-eyed accusations are true; it doesn’t matter to him whether they’re offensive. All that matters to him is casting an illusion his supporters want to believe in.