Because it gets broken and then it cuts your foot if you unknowingly are barefoot walking over it (that’s a fun scar) ruining your day at the beach because you don’t want to get saltwater in your cut because ow bleepin fuck.

They refer instead to the butthole, though

You and I are sheltered I guess. I thought it was to prevent people cutting their feet at the beach (presume that’s not uncommon).

The butt thing does make it funnier though.