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 am agree with you, selling bitcoin is not a stupid decision it is a actually a wise decision. All of us has a different decisions. I think the person sell bitcoin even though its price is down is a brave decision. why? because its really tough decision to let go your bitcoins in the lower price, forsure he has a reason for it. We all have different decision so lets respect it. And actually, if you think that you are satisfied in the bitcoin price today then sell it, because you did not know if it will be high or if it will be down again.