Here are some generalities that I am told apply in the American legal system. There is a good chance that something similar or equally undesirable may lurk in the legal system where you live.
You are personally liable for the injury and damage that you inflict on others. That is cost of medical treatments, loss of income, pain and suffering, potentially legal costs for both parties, etc. There is also a good chance in the USA that you may not have insurance to cover you in such instances. Some homeowner policies may cover this but many may not.
So, as a reward for lack of knowledge, overconfidence, indifference or an acute attack of "kiteboarder attitude" that results in injury to others you may have a major legal ordeal to go through AND a major economic hit that could cost you a devistating amount. that could clean you out or take a major bite out of your resources. Add to this the guilt of harming others which may actually sink in after the smoke has cleared.
The days of "I didn't know" are gone, indifference to the safety of others will slam home in terms of complaints, lost access and some notable legal settlements in time for some unlucky, careless kiteboarders. One guy has gone to jail and there may be at least two lawsuits underway at present and NOT in the USA over incidents involving bystanders.
IF YOU USE DISTANCE
you will reduce the odds of such a calamity substantially. If you want to ride close in and show off you may be tossing your assets and peace of mind to the four winds. Not worth it. What about drag racing down the street where you live, is that OK? If not, why not? More about this from an actual attorney at:
Time to get serious guys. Shred hard and go nuts, far AWAY from bystanders and USE A KITE LEASH.