The argument against bonding goes that they are not "exposed" steel members.
It is clear anything attached to them like the dryer vent can or the medicine cabinet is exposed.
They use plastic boxes because they are a few cents cheaper and faster to install.
Yup they just screw the mounting flange to the steel framing with drill screws.
If they used a steel box for the dryer, the stove and the bathroom vanity outlets they would have most of the steel bonded.
