I guess there are two 'modes' of the sign. One is advisory - when turbulence is a risk, and one is mandatory - just prior to landing and takeoff.
If people were walking around at 50 feet above the threshold I dare say the hosties would not hesitate to give someone a bollocking. Otherwise the FAA would soon be having words with the airline.
At other times including early in the decent/late in takeoff, it's not such a safety issue so not upsetting passengers is more important. The airline is covered from a litigation point of view.
Lots of people see rules as something to be broken and they are often the more successful people in society. Airlines tolerate these people because they bring in revenue.