I remember watching a video of a guy doing that to a display phone at an Apple Store.
While that action was definitely not a good idea, it did encapsulate just how polished that jailbreak[1] was. The UX was identical to an App Store install page of the day. You tapped the price "FREE" and then tapped "INSTALL" and the phone would appear to install Cydia as though you had just used the App Store to do it.