How's this hypothesis:
The temperature of the Starbucks fluid (coffee/tea/milk) is 150-160 degrees when poured into cup. To steam sterilize equipment in an autoclave, it's 120 degrees, under pressure, for 20 minutes. Washing hands in the sink, under the tap, is 100-104 degrees.
If the reusable cup had cold-germs on it, there a greater chance the 'dirty' cup would be sterilized by the temp of Starbucks coffee versus hand-washing the cup in the sink @ home.