Actually, that IS the point, for all the reasons I've stated above.
If I can't tell whether the sticker is authentic or not at the moment I hand over my money, then the sticker is pointless, UNLESS it is being used to determine if products are eligible for manufacturer service. The goal is not to prove authenticity in the abstract. In theory, the goal is to keep people from buying fake products (where Sennheiser doesn't make any money), so that they will search out authentic products (from which Sennheiser does make money). But since you can't see the sticker in the packaging, wouldn't know if it was authentic even if you could see it, and there aren't any fake HD600s....
THE.
STICKERS.
ARE.
COMPLETELY.
POINTLESS...
...except for marketing.
And when was the last time you checked the hologram sticker on your install disks for anything? Do you have any idea what the hologram on those even looks like (or is supposed to look like)? Those stickers serve a completely different purpose. Their purpose is to show you if someone OPENED the paper sleeve the install disks come in. If they did, then someone might have installed the software on their computer, put the disks back in the sleeve and then resold it. You would be buying the disks, but (in theory) your license code or whatever wouldn't work. Those stickers are supposed to work by showing if they have been tampered with. The stress of opening the paper sleeve is supposed to cut/ruin/distort the security sticker.