From experience with government operations, the answer is likely because they knew it would be a lengthy, drawn-out process to get rid of someone problematic.

Government program means apathetic mid-level management, lawsuits for civil rights violations, and all sorts of red tape.

In most scenarios I'd envision, the person being thrown out is incentivized to try everything they can to avoid getting tossed. Which includes lying and concealing behavior.

Or, they could just have a clearer rule that allowed them to remove people with unaddressed substance abuse problems. Much easier.