The reason is that we have to provide some power source for the charging ho lder of a portable device, either attached or detached to the A/C plug. If we supply with attached plug, we have to worry about the A/C plug itself. USB A/C plugs are widely available and cheap. But because USB ports are a lso available on PC and laptop, we have to worry about them.
However, a properly built PC/laptop USB port should limit the current and/o r disable the noisy port. USB devices would not have been so popular witho ut this simple and fundamental safe-guide.