Are Android TV Boxes legal in the US?

Yes, Android TV Boxes are legal in the United States.  Android boxes will never be illegal but the software that comes installed on some could be illegal.  Android software that comes installed on most boxes is the same software you probably have on your cell phone.  There is nothing illegal about that software or even Kodi for that matter.  What is illegal is the 3rd party software that allows you to stream and watch movies for free.  A lot of times there are boxes that are described as fully loaded which means it will have the 3rd party software necessary to stream the latest TV shows and movies.  You can buy Android TV Boxes that are not fully loaded and just install what you would like after you receive them.  In fact, a lot of people prefer the boxes with minimal software so they can customize them with various softwares and skins.  Hopefully this clears up any confusion about what is and is not illegal.