Between the Peaks Souvenir tumbler

So I can bring an empty refillable water bottle to the festival, but I can't bring an empty reusable plastic glass? If I drink alcohol I am required to puchase a stainless steel "between the peaks" branded tumbler! For me this will be a single-use item anyway as I will never use it again. So what's the deal?? I'm replacing a single-use plastic glass with a single-use metal glass? How is this helping the environment??