I've always believed that these stickers cannot be enforced. Most countries now have laws that enable a guaranteed time limited warranty against manufacturing defects, and this is not something that a manufacturer can simply void away by making the sticker a condition of the warranty.
Most warranties are written to limit your rights, but cannot void your rights completely. The sticker might be legal only as a condition of a
Full Warranty, sometimes also known as a
Voluntary Warranty (i.e. the written warranty that might provide additional warranty protection above what your state or country already provides for in the statutes).
I think this was challenged once with ASUS in America, and maybe there are other examples in case law.
* In the USA, there is the States' Lemon Laws, and the Federal
Magnuson-Moss Warranty Act that mandate implied warranties that cannot be taken away.
* I thought Australia has the ACCC that covers this?
* Canada has the Canadian Competition Act that prevents something called "tied-selling" that ties you to approved service only, or approved parts, or approved anything.. i.e. breaking the sticker to make a repair or service it with something or at some shop that they didn't approve of is perfectly legal in Canada and cannot void your warranty.
Furthermore, even the statutes sometimes have a clause that says a manufacturer is perfectly within it's rights to void your warranty if they can prove that you DID NOT PROPERLY MAINTAIN IT. How can you maintain it if you cannot open it up to clean it? For example, if you use the device in a dusty environment, you'll want to open it up to clean it and prevent failure. If it fails because you couldn't clean it, you might try to get a replacement under the warranty, and they might be able to legally deny you. However, in order to maintain the device, i.e. clean the dust out, you have to break the sticker, and then the manufacturer says they will deny you because you broke the sticker. That's clearly a catch-22 situation that you cannot avoid.
PS: IANAL