The only link between the two situations is the words warranty and cosmetic. If you think about it the two situations have nothing to do with each other.
Besides please explain (give examples on) what you mean by "cosmetic change"?
Depending on what you do to it, you could alter the function of the machine as designed. For example, if you put a sticker over an exhaust fan and your computer overheats and needs service, it's your fault and they won't cover that. There are any number of other scenarios, but the point is that Apple doesn't want you altering the machine to potentially disturb the computer from the way they designed it to run.
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Besides please explain (give examples on) what you mean by "cosmetic change"?