Actually bitcoin is like others coin so its price can be high and low. So if people see that the price of bitcoin is high so they can sell. They can save something for the future also. There is nothing as stupid decision.
I agree with you. Selling your own bitcoin doesn't mean your stupid. It is yours so whatever decision you make is up to you and that is not stupidity eventhough you will lose it. It is normal for people to sell it especially when the bitcoin's price is high and greater than the others. Nevertheless, selling bitcoin when the value of it is low then that is lack of strategy but not an unintelligent act.