Are travel mugs "unsanitary?"?

On days that I have cles, I rely on the Barnes and Noble cafe to supply me with my tea. On long nights, I may go through two or three disposable cups. Multiply that times two or three nights a week and that's a lot of waste. They do not let you use the same cup (including travel mugs) for hot water (or coffee/espresso drinks) because it is unsanitary. This was particularly irratating after I purchased one of their travel mugs and they would not let me reuse it that night. I told the manager that I find this very wasteful, but they insisted it was their company policy due to sanitation reasons. Is this a legitimate argument? This cafe uses steel carafs to heat their milk and they do not stir their drinks (for good or bad). I order tea, which does not involve touching anything to the cup.