When viewing through the binoculars, if you change your stance the character will:

1. put away the binoculars
2. pull out primary weapon
3. change stance
4. put away primary weapon
5. pull out binoculars

You are also unable to change to any of the sub-stances when looking through the binoculars. (CTRL+W or CTRL+S etc)

You should be able to change stances without putting away the binoculars.