How so?
Because there’s no way to win that argument. ;)
How so?
The non sequitur aspect of it is this. Because Alcohol is legal, Pot should be legal.
The one statement does not follow from the other. The one statement is not a demonstrable proof of the other, and cannot be derived from the first statement, ere it "does not follow" and is a "non-sequitur". (Sequitur means in sequence. It is non sequential, meaning it is not part of a logical chain.)
It makes as much sense to say "Because Alcohol is legal, Opium should be legal." That also does not follow.