I think people are making mountains out of molehills (or cairns out of pebbles, ha!) here.
There are some largely undisturbed habitats with animal populations that rely on rock cover (such as salamanders in fairly unaltered mountains). In those places, people should be encouraged not to disturb the stones.
There are also many habitats that don’t really have such a delicate balance in place or are already heavily disturbed, where stone stacking isn’t a big deal. I would not say that doing so in such situations is pointless or even overly destructive. Some people might be doing it for social media clicks, sure, but what about people finding their peace in nature, doing little zen things? I don’t think we should scold those people.
A “perfect” environment for most species is one in which humans are never allowed to intrude–but if humans are never allowed to interact with nature, they forget why nature is special, and why it should be preserved. Look at all the people in urban centers who are blown away by the beauty of nature when they get to visit these places. Explaining to them why they shouldn’t disturb at risk environments is one thing, but if you scold them for stacking stones at an already heavily disturbed park or beach when it’s something contemplative they’ve found to do in a more natural setting, you’re going to likely ruin a chance at gaining new people interested in protecting nature.
It’s like the old bird feeder argument of what’s being gained vs lost in terms of interfering with bird behavior and possibly spreading diseases vs getting new people interested in loving and protecting birds. Balance and loving guidance is key. Just as the general view with bird feeding is that the best way forward is educating people about when to feed and how to clean feeders to make sure birds aren’t overly affected while still letting people enjoy feeding them, the conversation on rock stacking should be educating people on why some areas it shouldn’t be done, while not making a big deal over places where it really doesn’t matter much.
Everything on earth impacts the existence of something else negatively in the short term. Caterpillars swarm trees. Deer chomp down the undergrowth. In regulating nature we don’t try to stop these things, but rather make sure they’re done in balance. We don’t eradicate caterpillars or deer, but we try to ensure they stay in the right number for their environment to balance out between the good and bad they do. I think we should offer our own species a similar kindness. Even if stacking stones at a disturbed urban park is still a “disturbance,” we shouldn’t try to negate human existence out of the picture, but rather make sure we do just as much good as harm, if not more. If the person who learns to enjoy nature by finding peace in stacking stones at the park goes on to plant many trees and campaign against ocean dumping or so on, I’d say that’s exactly done. Lecture them into giving up all hope by telling them every last little thing they enjoy doing is harmful to nature, though… and you lose more than you gain.