Since there is no negative outcome to hitting the button multiple times, the question becomes really more of, "why wouldn't you hit it more than once?"
It would only take using a calculator with this behavior one time to create this behavior.
Think of it this way....
A person who has never used a calculator that works this way, one day uses a calculator that does. After some initial frustration, he learns that to clear he must hit the button twice.
He continues the rest of that day. Every time he hits the button only once his calculations are messed up (negative feedback) every time he hits it twice his calculation are correct (positive feedback)
The next day he uses a calculator that doesn't do this. However the behavior is already learned. But now whether he hits the button once or twice doesn't matter: both give positive feedback. Since both give positive feedback there is nothing to promote unlearning the behavior.
Now we continue on days or weeks later, he uses different calculators but doesn't really differentiate them. Hitting the button becomes like touch typing, a simple learned response.
When I type I don't think of the letters that I type or even how I move my fingers as I type. I just think of the words and brain an my fingers know how to make the letter appear on the screen. When I hit clear on a calculator I don't think "find the clear button, press the button x number of times" ... no I think "clear" and my fingers press clear (however many times they do.)
Now here is the really interesting bit. Say after awhile he does stop hitting the button twice. Ok well no problem on his normal calculator. However every time he picks up one of those other calculators, he's going to get negative feedback and the double tap behavior will again be reinforced.
Since he doesn't really differentiate these calculators when he picks them up, eventually his brain will determine that the cost in terms of effort of not hitting the button every time is greater than hitting the button unnecessarily. So... he just presses the button twice all the time.