Weight is definitely a huge concern with helicopter ejection seats, as is cost, but operational use is also important. Generally the helicopters that could use ejection seats are flown aggressively and close to the ground. By the time you decide ejection is a good idea, then initiate the sequence, then wait for the blades to clear, then fire the seat out...you've already crashed.
Sure if you're at 500' and pull handles early you'll be alright, but under what emergencies would you choose to trust the seat and parachute rather than flying it yourself?
I'm not saying it would never be used, just that its value diminishes quickly when you realize the limitations.
Back to the original topic:
Interesting that Jack Stall is not allowed in US Military helicopters, but that doesn't mean a pilot cannot fly those machines to the breaking point, just that the breaking point doesn't include Jack Stall. There are limitations to follow on all machines, if high speed, high 'g' flight isn't allowed and is the only condition where Jack Stall occurs then that shouldn't be a problem.
To call it a design flaw is like saying any helicopter that can intentionally be oversped, overtemped, etc. is flawed.
Designers have to meet certification requirements and prove safety of flight, but they do so knowing there is a pilot in the seat and knowing that they can expect the pilot to follow rules. If that weren't the case then there would be no limitations section in your AFM/AOI/PH because the helicopter would handle that for you (maybe we'll build like that one day, but not yet).
I've seen Jack Stall and I don't want it to happen in the wrong conditions (low level), but I continued to fly that same design and occassionally had to fly it close to Jack Stall conditions.
It's not a flaw.