Is helping American companies by working to develop their computer systems considered assisting countries that occupy Muslim lands and support Israel? And is this line of reasoning Islamically sound?

If your field of work is permissible and does not involve aiding in what is forbidden, then there is no harm in it. Its permissibility is not affected by the company being owned by a state that may fight some Muslim countries or assist those who fight them. However, if your intention is to leave work as part of an economic boycott, then refer to the mentioned fatwas regarding that.