beach balls just cause havoc bouncing around and potentially knocking things over. they're a nuisance.

blankets tend to want to be laid out on the floor for people to sit on which takes up a lot of space causing havoc for foot traffic when people are not expecting to have to step over someone. also, they can be used to start fires. these are the same reasons they are no longer allowed at outdoor concert venues for specific types of shows.

People toss beach balls around and it's distracting.

People use blankets to claim more space than they need and sit when they should be standing.

Both of these are perfectly normal rules for big gatherings.