Why is only intentional action considered as Karma, which gives rise to corresponding consequences?
What you say or justify do not count. What counts is your volition.

For volition to give result the volition should have any of the 6 roots (3 wholesome - alob, adosa &amp; amoha + 3 un wholesome - lob, dosa &amp; moha ). When you accidentally step on an insect you do not know of its existance non of these roots arise in your mind, as you do not have any intention to kill. You know you are steeping on insects but as iterated before you do not have the intention to step on them, hence there is no un wholesome roots in your mind to create the Karma.

When you do rob someone what ever the justification you are motivated by one of the roots (loba). Even satisfying your hunger is due to loba. What ever the justification there you know and have the intention. Hence there are Karmic results. Every wilful action need intention, even an action which might not generate any karma like lifting your hand. Your body will not sleep walk you into robbing without any intention or thoughts.

Also I asked this related question which might complement the above question: Where in the Tripitaka completion of the formation of Karmic result is mentioned?