Can an institution rescind a job offer

Your employer may terminate you for failing a drug test, since the drug test shows the presence of an intoxicating substance in your blood stream. It is possible that if you have marijuana in your blood stream, that you are still somewhat impaired or under the influence at work; they don't need to accept that liabilty risk, the same as they can terminate employees for being potentially impaired by other legal drugs (e.g. legally prescribed opiods; alcohol) at work. The problem with marijuana is that it lingers in your body and the tests are not sensitive enough to tell how impaired you are or when last you ingested or smoked it--but the employer does not, as stated, have to take the chance that you are impaired. 
The medical marijuana card means you won't be prosecuted for using marijuana; it doesn't guaranty that you can work while taking it.
Unless you had a written employment contract--not just an offer letter or welcome packet, etc.; an actual contract--guarantying you the job for defined period of time (e.g. a one year contract), you were an employee at will and could be terminated, or have an offer rescinded, at any time.
(Also bear in mind that IL cannot change federal law, so marijuana is still an illegal drug under federal law. Any institution getting any federal money--e.g. Medicare or Medicaid payments--must be careful about employing someone who is violating federal law.)
Unfortunately, this was legal.