I think you'll find that the program being used to print receipts always includes the line for a gratuity. If you have already tipped or don't want to tip, simply ignore the line.
Or the marks could be feces left behind while in storage, or mold/mildew. We are becoming paranoid about everything. Stay home, don't use public bathrooms or, perhaps, bring your own toilet paper.
It sounds like another case of taking a cup full of sand and claiming a dune. This lawsuit should be dismissed until qualified independant researchers run a proper study with proper samples. Mom's Across America should pay for the research.