since some people use the terms casually without understanding that they have specific meanings, and since both phrases use English words that could be interpreted to mean something else. (For example, “free software” doesn’t mean software whose price is zero, and “open-source software” doesn’t mean software whose source code is published in the open.)
The Free Software Foundation can make whatever definitions they want, but they don't supersede regular English. That's not a problem with "some people" being casual, it's a problem with a small entity trying to claim a common term. The confusion is entirely their fault.