For White House staffers and even high level officials, there are strict ethical standards that are supposed to be followed. In fact, some of those ethical standards are codified in criminal laws. Among the most well known type of ethics crime occurs when an official accepts a bribe.
Generally, the ethics rules seek to prevent government officials and employees from making decisions that they have a personal or financial stake in. This breaks out into two topics: conflicts of interest and personal financial interests. Not surprisingly though, the rules do contain enough leeway, and the executive branch is vested with enough power, that even clear conflicts can sometimes fly under the radar. Additionally, using a public office to garner publicity for private industry, such as by making an endorsement, even if no money is exchanged, is prohibited.