This is a petition for a legitimate response that includes a good reason why marijuana is illegal while known more harmful substances are legal.
"...marijuana use is associated with addiction, respiratory disease, and cognitive impairment" from Gil K's response.
If this is how you're going to start your argument against every pro marijuana petition then how is it that alcohol and tobacco are legal? Tobacco products and alcohol are both addictive and both are harmful, even deadly. Marijuana is known to be safer than either of the two legal substances mentioned above, yet we are told its illegal because of the "dangers" it can bring us.
NOTE: This is not a petition to make alcohol or tobacco illegal or to even bring them into question, those are and should be personal choices too.