At December 2014, Daniel Waters (one of the screenplay writers of the movie) was asked about the seashells in Demolition Man and gave a decent reply: (warning: graphic language used!)
"There's a scene where Stallone has to use a restroom. I'm trying to come up with futuristic things you'd find in there. I was having trouble, so I called my buddy, another screenwriter across town, asked him if he had any ideas. Ironically enough that guy was taking a dump when he answered the phone, looked around his bathroom and said 'I have a bag of seashells on my toilet as a decoration?' I said 'Okay, I'll make something out of that'".
So, to sum this up nicely: the reason why seashells are used, and not something else, has two levels:
- The writer has a friend who had (maybe still have) a bag of seashells on his toilet as a decoration.
- When the writer called that friend to ask for ideas about future things in toilets, he was just using the toilets, thus the seashells caught his attention and he mentioned them.
Without any of those, we would have most likely gotten some boring thing like fancy bidet, which would be the obvious choice and wouldn't have raised any questions or discussions. :)