After doing some research on the US census website, my best guess is that, yes, tourists are required by law to complete the census. Though they definitely seem like an afterthought, since I couldnt find any direct reference to tourists completing the census. And also, while technically legally required, on the same page that explains that you are legally required to respond to the census, the government also wrote a whole section about why you should respond to the census.

So in practice, almost no tourists are going to complete a census form, and they will face no legal consequences as the census has basically no legal enforcement mechanism.

As for your followup - why would they want tourists on the census? - Well, because they want the best data they can get. They’re a bunch of nerds collecting data to do statistics on. Why would they just leave data on the floor?