There are two main reasons for this.
First, the Western companies may want to act ethically but may be unable to do so. There are many developing countries in which practices that would be unethical in the West, such as bribery, are simply expected. It is unreasonable, one can argue, to expect companies not to “play by the rules” of the country they are in.
Second, it could be unreasonable to expect this due to competition. If there are many companies doing business in a given country and some are acting unethically, it is almost impossible for the rest to be ethical. If they were, they would be unable to compete with the unethical firms.