Because people are idiots. I really can't say a whole lot more.
Also consider that many "Small Server Owners" haven't put a lot into their server, so they can't be expected to behave well. The reason that owners of large servers tend to be more respectful, mature, and well-behaved is that they've put a much larger commitment into their server. If a lot of people play the server is probably paid for, dedicated, has good builds and staff, etc. If an person puts that much work into anything, they won't risk screwing it all up by acting like a jerk.
