> idea of making top surfaces slanted in public spaces to keep people from placing empty drinking cups and such on top of them

That's a low-key example of hostile architecture, isn't it? Other than preventing people from leaving cups around, this decreases the utility of public space. Everyone sometimes finds themselves suddenly needing a flat surface other than pavement for a couple of seconds or minutes. Say you've grabbed multiple coffee cups wrongly, or packed your groceries badly, and need to repack. Or something fragile you carried broke. Or you need to make a note on the spot. A raised flat surface nearby is godsend in such situations.