Make opening their products accessible so that people can do it without damaging what they own. PCs are like that. Car parts have no such warranty stickers (that I am aware of).
Stickers are much more cultural than actually preventing damage. Furthermore, you can have stickers that decay over time and void your warranty without you even touching them (those nefarious apple humidity stickers for instance). It is entertaining to see Steve from gamer's nexus bashing warranty stickers at every opportunity.