I agree that the OP doesn't have to justify--but I also agree that there's a special annoyance factor involved when one has *just* completed a cleaning project and someone comes along and deliberately messes it up before you've had time to enjoy the cleanliness.
For me, it used to be the bathroom sink--I have a "thing" about toothpaste spit. When I was still married and homeschooled the children, I would clean our bathroom once a week and I'd get that sink all shined-up. My children could manage to brush their teeth after lunch and wash the toothpaste down. But my now-ex used to come home from work, brush his teeth and leave toothpaste spit *all over* the sink and leave skidmarks on the toilet seat. I'd ask him if he could make the effort to wash it down so that I could have at least 24 hours of clean sink. Never happened. Once I kicked him to the curb (not over the toothpaste spit and skidmarks on the toilet seat), I haven't had to deal with toothpaste spit in my sink since. Same thing with my kitchen floor--in that house, one entered through the laundry room (where five of six of us took off shoes, guess who didn't) and from the laundry room through the kitchen into the rest of the house. It was as though as soon as he came home and saw I had cleaned any particular thing, he *had* to make a mess on it (tracking snow across a dry, yet clean, kitchen floor, making a PBJ and smearing PB and jelly on the counter and leaving it *and* leaving the jars open right where he used them). Funny how, once we removed one person from the household, the work load of keeping the place clean lightened by more than 50%.
Yes, floors are for walking on, and they tend to get dirty as a result. But when the OP says "just finished" I'm picturing a floor that may still have some wet spots and she hadn't yet had time to stand in the room doorway and smile over the results of her efforts. People who live in the house generally know (unless they're rude boors like my ex) that when some cleaning work has just been finished that it's not ok to deliberately mess it up. Someone who doesn't even live there dismissively intending to make a mess and act like it's no big deal to just "wipe up after"--that's just astonishingly rude, particularly given that the OP mentioned that the aunt wouldn't have had an issue doing that w/ someone who is "on her same tier" (i.e., a sister instead of a niece) and that aunt had plenty of other bathroom options that wouldn't involve imposing on anyone.