Drugs should be guilty until proven innocent. Comments like these attempt to make them innocent until proven guilty. History has shown that there are very few "safe" recreational drugs.
This attitude is one of the reasons it was so hard for public opinion to change on tobacco usage.
"My mom died of cancer from smoking tobacco"
"Sorry about your mom, but I want to point out this is anecdotal. Can tobacco lead to cancer in people who are prone to it genetically? Or are people that are genetically prone to cancer drawn to tobacco? Or does it affect everyone? Lots of big unknowns here, so let's play it safe and not jump to any conclusions about the harm tobacco potentially may cause."