I've seen those stickers on hard drives. I always assumed that maybe the internal mechanisms were in a vacuum or super-clean and opening the case would allow air or dust into the moving parts.
Considering the nature of hard drives, I think that would fall under damage caused by opening.
My understanding is that they are not in a vacuum, but they are super-clean. The air/gas inside is an important part by preventing the read/write head from touching the platters as they rotate. If a consumer opens it up, then dust is introduced which will cause problems.