well i always had this argument for jailbreaking

When you buy a PC with windows on it, and you change the OS to linux, does that void the warranty of the laptop, even though you havent disassembled the device in any way...
yes, using a third party OS opens the door to security vulnerabilities,etc but thats a risk the user is taking.
going back to my laptop example, if i lose data on linux OS, i cannot hold the laptop manufacturer responsible in any way.

but then again, some manufacturers are against third party software coz it wont allow you to buy a new device soon enough. custom roms add new features and even support the device on the latest software even though the device is no longer supported by manufacturers.

i used my old HTC for 5 years before upgrading.

The fact is that companies do this to protect themselves, because what happens if someone jailbreaks the phone and breaks it? Apple should not be responsible for it. Same thing with the void sticker thing, what is stoping someone from opening a console or computer breaking it and then have the company fix your error? Companies need protection from consumers just as consumers need protection from companies.