Why is only intentional action considered as Karma, which gives rise to corresponding consequences?

Consider a situation (just a silly example) like this: Suppose I'm walking on a road or grass field, and I know there are living beings like ants and other insects lying down on the surface, but I don't have any intention to kill any being while I walk there to reach my destination. But, as I'm walking, consequently, many little beings are killed by me by stomping on them, though I don't have intention to do so. Buddhism says that only an action done with intention will account as a Karma that will affect us accordingly.. Referring to this and the situation I gave for example, does it mean that the killing of insects without intention (though I know I'm going to kill them) isn't a bad Karma? Will I not face any effect due to that action I did unintentionally, but KNOWINGLY? If that's the case, why can't I rob a house and say that I just did it to quench my starving stomach, not to hurt anyone? Isn't the consequence of doing an unintentional action considered as a seed that can affect oneself? Why? If you say I'm wrong, then how is Karma in Buddhism different from that of Jainism, where every action is a Karma?