With big festivals with thousands of people gathering in park or square, you can expect this in Europe. Problem is that you buy a drink in a plastic cup or bottle, maybe a sandwhich or fries with it, and you need to dispose of the plastic or paper. Where do you leave it? There simply are not enough trash cans around or nearby. In a big crowded place it's not easy to move around, you can get lost etc. So people throw their garbage on the ground. By the way, all available trash cans are full and have a pile load of garbage next to them. So even if they are full, people still use them to keep the trash together.

I don't see the problem here. Afterwards cleaners come and clean the square, streets, whatever. It's part of the game.

On normal days, people here keep the cities, parks and streets rather clean.