question about chicagos pot laws

I was arrested for an eighter in Chicago and I have a question I hope someone can answer for me. why was I arrested if weed is decriminalized in chicago?I was not in a school zone or smoking. I had the weed in my pocket and undercover cops rolled up on my buddy and I as I started my car.

It more discretion than anything,are you over the age of 21? and if you are over 21 the only other reason i can think of is you were in a car.(DUI)

I am 20, the reason I ask is because they didn't tell me exactly y they were arrested me (i assume it was the weed lol)nor did they read me my rights. I'm just thinking about if I want to get a lawyer because if its the ticket, witch its supposed to be) its only 200 to 500 dollars witch is less than a lawyer

You was arrested because you are under 21.You must be 21 and over with valid ID,and for people who can't produce an ID and or under 21 are automatically arrested.
Police Discretion Is the key part in our decriminalization laws.

What district? did this happen in?