

I would never advocate or incite for imminent lawless action. I would advocate for a change in public policy that included things currently held to be illegal.
As long as it makes a difference, the first amendment protects your right to advocate for changes to law and policy. You just can’t tell people to right now go and break a specific law if you have reason to believe they might.
“People should shoot maga fascists dead in the street and hang them from lampposts” is protected because it’s saying what society should do, not actively calling for people to go do this.
It’s from a time when you bought undried and planned wood rather than dried and planned like we typically do now.
It’s less a quirk of the imperial system and more a quirk of the lumber retail system, which is older than the metric system.
The biggest difference is that in places that use dimensional lumber and the metric system the pattern is to sell by actual dimension, rather than nominal. So a wall stud might be 45mmX145mm, or 63mmX75mm for a rafter, depending on your country.
Most north American hardware stores also sell by finished sizes now.