The problem is in order to wipe it, or do anything to it they have to send someone to site, and they dont have engineers that work for nothing, nor vehicles that do not require any fuel or maintenance to get the engineer to your property, or any admin staff that work for free to organise it, and so on. To have engineer codes per customer would be a real ball ache, we have all thought about this over the years a code based on postcode or customer number. The thing here is, if this was the case and ADT gave out engineer codes to customers when they canceled their contracts it would not be long before the method of how the codes are generated is worked out, leaving all ADT customer vulnerable. ADT have a responsibility for all their customer, and less so for the ones that leave (they are no longer a customer)
Later panels can be programmed remotely and panels can be defaulted remotely, but this is a fairly new development, and probably not available when your panel was installed.
ADT are not try to scam you, they are protecting their customers, and you are welcome to pay someone else to disable it, or try to disable it yourself, or just leave it where it is and not use it