Recreational Landsailing is great fun, and part of the fun can be found in careering around on two wheels, throwing the yacht into tight turns, sliding sideways and spinning the yacht around, particularly if there is a crowd of appreciative onlookers nearby, in other words... showing off.
But... to the nervous onlooker you can appear to be out of control, they can feel threatened, and think their life might be at risk. That might be a bit extreme, but it is the reasoning behind the guidance that says you should give everyone 50m clearance. On smaller beaches 20m is sometimes specified and, if you cannot give everyone 20m clearance, you should accept that the beach is too crowded.
Do give folk a cheery wave - it never does any harm - and always be polite, regardless of the provocation.