You may also have heard that HitchBOT was destroyed only two weeks into its trip through America.
These actions are not random, isolated, or irrational. Instead they are coherent symptoms of a wider problem, indicating an unhealthy society.
In a healthy society, people share a common set of guidelines for behavior. They generally view other humans in a positive light, and act to take care of the helpless, because they remember other people taking care of them in need. This readily extends outside humanity to animals and even inanimate objects. These folks therefore respect public property and look after things in the commons, so the overall rate of vandalism tends to be low.
America is not a healthy society. Too many people are poor, unemployed or underemployed, unhealthy, unhappy, etc. because of many and widespread decisions that have degraded our quality of life. They feel insecure, angry, and frustrated. Their efforts to fix this are met with derision and often violence. So when they see an unguarded resource, their instinct is to strip it -- because that's how people treat them, because that's how wealthy people act and they wish they were wealthy, because they don't know when they'll see another chance, because they're fucking tired of starving for everything all the time, or maybe because they want other people to suffer as they have suffered. So too, when they see someone or something helpless, they are prone to lash out, because they can only express their anger downward in the power structure, and they feel that's better than nothing or they are just plain out of spoons to resist temptation or they don't know how to behave better because violence is all they've ever known.
This is why we can't have nice things: because hurting people hurt people, and the cycle continues.
To consider the ethical aspects: Yes, you can steal from a Little Free Library. There are several reasons for this. One is simply that it has a usage agreement of "Take some, leave some." Emptying it violates the rules and therefore constitutes theft. Another is that items can be placed in public or semi-public space without being abandoned. Citizens have a responsibility to treat these with respect and not steal or destroy them. Examples include loose equipment in public parks, city-owned bicycles, and BookCrossing books. Then too, there are use rights for commons. Among the most common rules for that is: You may not take the last one of anything, unless someone else took the second-to-last item and there is now only one left. This type of social agreement is how groups of people manage resources which are held in common rather than privately owned. But when society allows certain individuals to suck out the value from the commons -- for example, letting corporations clearcut national forests, strip-mine mountains, frack into aquifers, etc. -- then people start thinking it's okay and you see countless examples spread throughout everyday life, from things like this to people who routinely steal office supplies from work because their employers gives them no benefits and they feel no loyalty to someone who treats them so indifferently.
It destroys the bonds which hold society together, leaving us with a cluster of miserable, misanthropic hermits instead of a culture. What we are seeing is just the fallout from that.