This isn't a lack of intelligence, not even lack of mathematical intelligence.
First there's bound to be a bunch just glancing, thinking "oh, it's times 0, so it's 0". I do this all the time, and I know for a fact I'm around average+ on intelligence, and I was a killer at math in school, but I have a short attention span, and tend to do errors on easy tasks.
Then there's the group getting the right answer, but thinking it's a trick question, and not bothering to double-check, so they answer 0 not to seem like they fall for the trick.
Then there are the ones that simply forget how BEDMAS works. BEDMAS itself has nothing to do with math, it's an arbitrary system, math is logical. Understanding of math and remembering BEDMAS is un-related. Yes, it's sad so many don't know remember it, but most people that doesn't use math on a daily basis (like me) don't refer to calculations when we need to figure out something. And if we do, and forget how to use BEDMAS it's irrelevant, since we wrote it down ourselves, and thereby don't need BEDMAS to know when to multiply or add. BEDMAS is used so that I can write a problem, and anyone can solve it without extra explanations. As long as we can keep track of our own thinking BEDMAS isn't necessary to remember in everyday life.