Dropping food can lead to a menace from birds,
as reported here
So the fact that pastry is biodegradable is irrelevant. Picnic areas are supposedly regulated, or they should be, but the actions by others, training certain birds make them more unusable.
As for sitting on the seafront eating chips, not everyone puts their uneaten chips in the bin, and the smell from the wrappers can be enough to attract vermin.
If I want to eat fish and chips at the coast, there is a nice restaurant I can sit in and eat, out of the weather and away from the flying menaces.