I was told by someone smarter than me that most of the time when someone says, "I (we) can't", what they really mean is, "I (we) WON'T".
It sort of depends on their perception of the down side of making the statement.
It was said that the two words one never heard spoken in the Duma under Stalin were the words, "I can't", since that phrase was equivalent to a declaration of a desire to stop breathing.