So fine, fair enough. You say, "Excuse me," because people can't read your mind, after all, and maybe they could reasonably think you are in love with them but have some social disorder where you don't know that you shouldn't stare directly at people, which has to be more likely than the possibility that you want to use the frequently used machine behind them, and clearly the empty coffee mug you are holding is just a red herring clue they don't want to be distracted by. So you say, "Excuse me," which should clear everything up.
Then invariably they move from standing in front of the coffee maker to standing in front of the coffee condiments, which means that after you have poured your coffee, you are going to have to ask them to excuse you again, after which they'll probably stand in front of the garbage can so you can't throw your stirrer and empty sugar packet away. And there's a limit to how much you can help them out, because saying "Excuse me" is acceptable enough, but "How about you just stand in front of the stupid labor laws?" is considered patronizing.
You'll see the same thing in many other places, too, like the gym, where people will go pick up some weights from the weight rack, and then start lifting those weights right in front of it, because come on, who else is going to want to get weights? In a gym?