The difference is that CEOs and celebrities aren't public servants. It absolutely matters if people are spending money on the president's businesses to try and curry favor with him.
Wrong. When you pay a business for a product or service you get something in return. It is a stretch to say that a business owner owes something more to someone who already bought a good or a service from him.
In contrast to owning and operating a business, politicians regularly solicit and receive millions of dollars through donations. Politician like Obama were asking for large sums of money from donators, even in his last term. Why should we believe that a business owner is more likely to be corrupt than a politician who is handed a wad of cash by a corporation for vague reasons through donation companies?
What you believe should happen is also irrelevant to the fact that Trump's voters knew that he would be running his businesses once elected, especially when he was elected for his second term. Trump made that very clear, and people supported him for it, especially in light of the malicious prosecution against Trump and his companies. The voters wanted this. That is called democracy, and supersedes your small leftist complaint.
We were doing just fine with the expectation that the president should not be making money on the side before Trump came along, and we'll continue to do just fine by taking the next step of codifying it into law.
Laws which are codified through Congress are the result of democracy and representative democracy. Congress votes on the matter, and people vote for congress members. In this case, the people have already voted on this.
The fact that businesses have existed long before the creation of the country and neither the founders of the United States, its many congresses, or its courts up to present have had an issue with this also shows that you are on the losing side of this and do not have a supportable argument. You would be better off with a list of examples of corruption which have harmed the country through this mechanism, rather than handwaving a potential one in contrived scenarios where people owe strangers favors from unsolicited money and without agreement that a favor is owed.