The barge was located about 400 miles downrange of the launch site in this case, so if you're landing in Lake Michigan then you're launching from somewhere around western Iowa/Minnesota or eastern Nebraska/South Dakota. When your rocket goes off course or explodes, you'll rain parts and fuel down on Minneapolis or Chicago or whatever other inconvenient population happens to be in the way.
"Oh, I'm sure I know a lot better than the accumulated centuries' worth of knowledge from actual rocket scientists, and I can come up with solutions to their problems based on no in-depth knowledge of the field and a minute's thought."
It was also an observation that this particular body of fresh water is an "easy target," apparently stated without putting any thought into what makes a target easy or hard for this purpose.
