It appears they have the proof they need. It is apparently up to you, to prove you did not use marijuana. Here is are excerpts from Missouri Dept. of Corrections Whitebook guidelines for granting probation;

Section 559.021 (1) RSMo. reads: "The conditions of probation shall be such as the Court in its discretion deems reasonably necessary to ensure that the defendant will not again violate the law. When a defendant is placed on probation he shall be given a certificate explicitly stating the conditions on which he is being released."
also
The specific conditions of your supervision may be amended or deleted or additional conditions added at the discretion of the Parole Board or your Sentencing Court.
and
As a part of your supervision, you will be expected to undergo urinalysis or other types of drug testing on a random basis. If drugs are detected, this will also constitute a violation of your supervision.
It appears they deem your being a pot head falls within their discretion. You should have read the terms you agreed to and asked questions, before signing on the line. I know, you knew they would not grant you probation if they knew you had already been breaking the law.