I checked to see about medical weed in Michigan.
This article from the Detroit Free Press from October 15 provides good background and perhaps places the TSA clerk's comment into context.
Apparently, there is a new law going into effect on December 15 which expands the availability and licensing of those in the medical marijuana industry in Michigan. They also appear to want to fix the current flawed legal framework One aspect of the transition is:
One of the big concerns for the people attending the meeting will be the transition time between the beginning of the application process and the actual availability of medical marijuana, especially since the state has said that they want existing dispensaries to close during the licensing process. And it will take licensed growers about six months to produce a medical marijuana crop.
<snip>
Before the new regulations were passed last year, the 218,556 medical marijuana card holders were supposed to rely on the 38,100 registered caregivers, who could supply marijuana to up to five patients to use to help treat a variety of ailments. That model will stay in effect with the new regulations, along with five categories of licenses — growers, who can produce up to 500, 1,000 or 1,500 plants, processors, testers, transporters and dispensaries.
To start fresh, the state Department of Licensing and Regulatory Affairs wants the dispensaries that are operating now to shut down Dec. 15 and not open again until they get a license. And that has caused a panic among medical marijuana users, who have crowded licensing board meetings to tell regulators that the transition time will leave them without the medicine that they need to function.
So, based upon what the OP overheard and the article's facts, perhaps a conclusion is that one of the clerks is a medical marijuana user concerned about the gap in availability discussed in the article. I could be wrong, but I can't imagine that a legitimate medical marijuana user would be physically able to be a TSA clerk. We also have no idea if the clerk was obtaining medical pot legally or not. Perhaps they are a grower who sells to a dispensary?