It's not controlled by ADT at all it's yours. The payment is for an engineer visit to remove it for you, not release from contract. Anyone concerned about security is unlikely to sell there used hard drive even "wiped".
Remote panel wiping would be another backdoor that you would not want as a customer in a system, if the customer wished to use the system while they found another maintainer they would have no system if the outgoing maintainer wiped everything on cancellation.
Honeywell maybe the manufacturer but they don't program them, each system is bespoke. You can purchase a computer with it's software (OS/BIOS/EFI) and then make your own programmes or languages and have there own licenses for example.
Yes we are talking about tiny embedded systems but its still similar. The panel is the barebones and useless in default state without an engineer knowledgable to programme it for that systems design, you purchase a "security system" from the alarm company not a DIY kit or individual components.