It must be remembered that the CDL is a secondary lock; fitting internal handles would be very difficult from a risk point of view.
It wouldn't have been particularly difficult to integrate into it the inside handle from the RIC door which is disconnected from the door electromagnetically while the train is in motion. (It reconnects when it drops below 5km/h, but we could easily have tied it to the CDL power supply if preferred). It being electromagnetic also means that a power failure means it releases, so it "fails safe" in terms of being able to exit the train in emergency, something which we don't seem to need to protect too carefully about given how many trains (even newer ones e.g. all 730s) are running around with the break-glass in the egress missing so it's no harder to trigger it than press the open button. And of course magnetic CDL as used on a few Mk1 DMUs (the Chiltern bubbles and the Swanage mainline DMU were the main examples) also releases when the power fails and was approved for the mainline.
So in short it could have been done.
Accessibility would be another reason - leaning out and operating a handle is really quite awkward and many people physically couldn't do it, particularly as the Mk3 handle had to be moved all the way to vertical against a quite strong spring.
It's more to do with the Health and Safety at Work Act 1974. In particular, the wording "It shall be the duty of every employer to conduct his undertaking in such a way as to ensure, so far as is reasonably practicable, that persons not in his employment who may be affected thereby are not thereby exposed to risks to their health or safety."
Interesting, thanks. I do support the HASAWA, but not because it keeps people safe from being stupid (though it does that as a side-effect), but rather so companies cannot compete unfairly with other companies by implementing inadequate employee safety policies. If it didn't exist, companies who didn't keep their staff safe would be cheaper to run, thus cheaper to the customer and thus would come out on top. Outside of health and safety, you can see how this might pan out by looking at the US, where companies with employee-unfriendly practices (e.g. poor holiday entitlement) tend to do well because their staff cost is lower. That's why it makes sense for the law to require companies to ensure safe practice for employees.
When it comes to customers I guess you want a similar thing - you don't want companies to be able to compete unfairly by making the situation unsafe for customers, e.g. using unroadworthy buses to transport them.
However it is a challenge to work into that the principle of personal responsibility. The Merseyrail cases are interesting in this regard - I would certainly say that if a drunk falls down the side of an already moving train with nobody in a position to stop it that it's entirely their fault, but equally if a member of staff dispatched a train with someone leaning on it then the member of staff had the choice not to do this and thus was at least partially at fault (as the Court decided). With regard to windows, I think I'd say that if a member of staff saw someone with their head out of the window they should probably tell them not to and make sure they were clearly understood, but then if they persisted it's on them. But you're right that the law doesn't see it that way.
