Earlier quoted context omitted.
In the US, the FCC has made it clear in recent years that “warranty void if removed” stickers are illegal. Manufacturers can only void the warranty if they can show that any defects are caused by your tinkering.
Well somehow I missed that (and also I am from Poland which means the sticker is still enforceable here). But being amateur electronics engineer myself I actually agree with the "warranty void if opened". Handling electronics safely is not trivial and if you touch electronics with your bare hand you can easily damage it due to static discharge. And there is going to be no way of telling who is exactly responsible for…
The FCCs argument is based on the notion that as long as you handle the device correctly, the manufacturer shouldn't be able to deny warranty _just_ because you opened it. You need to actually damage the device to lose out on warranty service.