Hi Bathsheba,
This is not a bug (although it might be confusing).
Even though a model is printed 100% successfully, the materials that do allow Print it Anyway will still show up in your cart (Designer only).
You don't have to mark the model if you don't want to (looking at your model, it won't be needed).
Which materials are eligible?
All Strong & Flexible Plastics, including Alumide
All Frosted Detail Plastics
Full Color Sandstone and Sandstone