that's just it as you said in the first part of your response; most end users aren't savy enough to make those decisions so they are doing it for them. they have to create things according to the largest common denominator (at least out of the box). If they left things wide open and everyone started getting hacked who do you think will get the class action lawsuit. The manufacturer of course so they have a duty to the customer but really they have a CYA factor involved.

part of giving the option leaves them more vulnerable. this could be from a reverse engineering part (information is key here. the more the better for hacking) or from end user screwing it up.

then they have to look at the target market for the device. there are way different rules and regulations when trying to sell cctv hardware to say a Casino or Government Institution then selling direct to consumer at fryes or something.

now from a developer point of view like you or I in most cases the manufacturer will work with you if you have a valid reason for 3rd party development. maybe you need a NDA or something like that but I find most of them are responsive to information requests when approached directly (and assuming they have the information. for instance in OEM/ODM situations many times the OEM company doesn't have the info).