Because it can be used to prop up any kind of harmful stereotype about groups such as genders, ethnicities, religious and political communities without having to engage the objections to unfair generalizations. In reality, the poisonous M&Ms analogy is a more manipulative version of the general tactic known as the “I know not all X are Y, but [flawed generalization]”.

And where might Junior, or the Trump campaign in general, be finding these poisonous memes? Well…

Here are some examples from the white supremacist website Stormfront:

The poison candy analogy is a favorite of the racist Alt-Right. Starting with the same set up as the Junior tweet, Stormfront has included as a punchline such winners as

So, it’s true that not all blacks are criminals, but apparently, all (or at least, most) blacks support criminal behavior.

and

So let me state again not all blacks are criminals but they are more prone to criminal behavior than any other race.

This brightly-colored bit of illogical idiocy masks real statistics and makes even the most unlikely event more frightening. Would you grab a handful of Skittles if even one would kill you? Well, thousands of people die in cars each year, so just think of each candy as a ride to the corner market. But showing you one bowl of candy and intimating that there’s a Skittle of doom waiting for you hardly represents the single fatality that occurs for every 100 million vehicle miles traveled.

Where did Uday Trump get the number three? That would be the total number of refugees — the total number — of refugees taken into the United States since 9/11 who have been convicted of involvement in any terrorist plot. And by the way, none of these men was from Syria. But the number of refugees admitted over that 16 year period is 784,000.

We’re gonna need a bigger bowl. In fact, you’d need about a 200 gallon container to hold enough Skittles to accurately represent the number of refugees who have entered the United States over that period.

And the analogy would still be a racist way of stereotyping the behavior of a whole group by the rare behavior of a very few.

Or, as Mars, Inc. replied …