Western companies have many ways to rationalize unethical behavior in developing countries. One should note that these rationalizations are not always ethical reasons, but instead ways to justify or put a positive public spin on acts that are inherently unethical.
First, companies argue that they are conforming to all local laws. It is true that multinationals dumping toxic waste in developing companies, producing clothing in sweatshops, or buying from contractors who treat workers in an inhuman and unsafe fashion or use child labor is absolutely legal in some countries. Thus, one could argue that these practices are "reasonable"; although, whether one should consider practices "ethical" is a different matter.
Second, often in the case of corruption, multinationals argue that engaging in practices such as bribery is necessary because other companies do it, and it is necessary to create a level playing field. They might also argue that it is "reasonable" in terms of how local cultures work. However, simply saying that other people do it is not precisely an ethical justification, as under the same logic, it would be acceptable to commit murder or child abuse on the grounds that other people do it.
Finally, arguments are often made on the basis that the business of a corporation is to make profits for shareholders and that this obligation overrides all other considerations.