In my country, you return glass bottles for re-use for about 15 euro cents. We don't see a lot of shattered glass bottles lying around...

Are aluminum bottles problematic? Sigg makes some great ones.

Aluminum bottles/cans are lined with plastic, to prevent the liquids acquiring a metallic taste. Beer cans are usually lined with BPA - which is bad news for your endocrine system. Sigg bottles have recently moved to a BPA-free lining, but it's still plastic.

I think their concern is others’ disposal of the glass on trails.

Edit: though I do dislike the idea of biking with glass bottles, especially in a backpack. A plastic container absorbs impact and can make falls safer. Not so much for a glass container.

I've lost so many tubes from glass fragments. Sometimes the tire even shifts around when going flat and pokes several more holes nearby. Still not as bad as people who throw bottles in the creek though.

Stainless steel is usually a good option - it weighs less than glass and is more robust.