Boycotting companies linked to an occupying power, and trading with non-Muslims generally

Answer

The point of a boycott, especially of products from an illegally occupied land, is (1) to inflict financial burden on companies profiting from illegal occupation, and (2) to reduce the economic power available to inflict harm on the Muslims affected. This is not an obligation, except where it is strongly suspected or known with certainty that supporting a particular company directly aids the harming of Muslims, in which case boycotting becomes obligatory and buying from them is prohibited. More broadly, business transactions with the People of the Book and polytheists are permissible except with "the people of hostility" toward Muslims -- jurists such as Ibn al-Mulaqqin, Ibn Battal, and al-Mazari note that selling weapons, or anything (including food, depending on circumstances) that strengthens those hostile to Muslims, is prohibited; Imam Ahmad b. Hanbal is reported to have held a broad prohibition on transactions that empower those hostile to Muslims.