As others have said, you have a very large misunderstanding in terms of what jailbreaking is and isn't which you should probably sort out before trying to determine the motives of people who do it. For example, there's nothing illegal about it. You may do more illegal things with it once it is jailbroken, but the same applies to a gun, or even a baseball bat. You can crack it over someone's head, but you can also choose to use it specifically for hitting baseballs at the local ball park.Why do people do this? Not only does it mess up your iPhone if you do it wrong but can get you in serious Illegal Claims. So what I'm wondering is why risk going to jail and lose your warranty? Apple REALLY need's to post Copyright Claims against Cydia as this is how most people do it. Or Apple could step up security and put a Handmade Trojan that it detects Cydia and destroys the iPhone permanently but the FBI/Apple will be knocking at there door.
Who think's Apple should do something about the use of Cydia and Jailbreaking?
I used to jailbreak frequently, but there's no longer a compelling reason for me to do so (I do miss game emulation, though). In the past it allowed me to add features I cared about like some degree of multitasking support. It allowed me to do things like work with Google Voice. The FBI would have considered my activities to be among the most boring things imaginable in that context.
Just because someone chooses to jailbreaks their device does not mean they're doing something wrong. (Although the people who use it to do things like pirate apps—even the apps they like—are asshats).