The stopper incorporates a pouring hole so you only have to undo the lid a few turns and pour. It wouldn't work with an external thread.Ive re found mine and am using it again.
Two questions;
why is the thread on the inside?
The coating on both my siggs looks more like a lacquer than anodizing. In the case of the patterned one it looks screen printed then lacquered over the top, rather than a decal.How do you remove decals while not damaging the anodised finish?
The stopper incorporates a pouring hole so you only have to undo the lid a few turns and pour. It wouldn't work with an external thread.
Water and fuel bottles benefit from an internal thread for the simple reason the diameter is less so it's stronger, and the thread is better protected from damage. From an engineering point of view it makes sense, and as mentioned, it's an insert.
https://en.wikipedia.org/wiki/Sigg
Some of the Sigg fuel bottles, and some various internally threaded vac flasks, have a pouring spout. Basically, all metal bottles and flasks I have are internally threaded. Keeps a seal well in metal I suppose?