The law says there is an assumption that caregivers will be engaged in the use of medical marijuana with their patients.
You misunderstand the law. RI medical marijuana use law, like that of other states with similar laws, does NOT allow the caretaker to use marijuana—only the patient for whom the marijuana is prescribed may use it. Being a caretaker is not an excuse or justification for the use of marijuana. Section 6.3 of the regulations merely states that a caregiver may not face arrest or civil sanctions for possessing the drug (within the limits allowed) or for assisting the PATIENT in using the drug. The presumption to which you refer states:
"There shall exist a presumption that a qualifying patient or primary caregiver is engaged in the medical use of marijuana if the qualifying patient or primary caregiver:
6.4.1 Is in possession of a registry identification card issued pursuant to the Act and these Regulations; and
6.4.2 Is in possession of an amount of marijuana that does not exceed the amount permitted under §2.8 of these Regulations. Such presumption may be rebutted by evidence that conduct related to marijuana was not for the purpose of alleviating the qualifying patient's debilitating medical condition or symptoms associated with the medical condition."
That last sentence is important, and reiterates what I said before—the purpose of the act is to aid the PATIENT with marijuana use. A positive test for marijuana use by the caretaker would be evidence of conduct inconsistent with that purpose, i.e. that the caretaker is himself/herself using the drug, which is not permitted unless the caretaker is also a patient allowed the medical use of marijuana. In short, that presumption does not say caretakers can use marijuana—it basically says that your possession of marijuana (which you are allowed to have to assist the patient) is presumed to be for the lawful use of marijuana by the patient unless other evidence suggests a use inconsistent with that presumption.
Thus, if you are not a patient prescribed medical use of marijuana, an employer can refuse to hire you or, once employed, may fire you for having a postive test for marijuana in your system. There is no reason why any caretaker should test postitive for marijuana (unless he or she is also a medical marijuana patient) because the caretaker should not be using the drug.