Did you try to scratch the natural titanium or the space black titanium? The space black titanium has a DLC coating so I wouldn't be surprised if you couldn't get it to scratch. I would be surprised if you weren't able to scratch the natural titanium.
I noticed faint scratches on the underside of my natural titanium watch which I find odd because of the location. There is only three things that could scratch that part,
1. My skin - We can rule this out.
2. A band - I've only used the sport band so even if I did fumble putting it on, it's too soft to scratch the casing.
3. The charger - Maybe but every time I place the watch on the charger the magnets connect it so it's not like I'm fumbling around with it.
Odd, makes me wonder if there are some issues with the way the coating is applied. All of the scratches on my watch are also perpendicular to the brushed finish, including the scratches on the underside of the watch.