Are these fliers for an event held by a third party at the local business’s premises? If so, I’d weigh up whether this a pattern of behaviour by the venue or by the promoter. And consider whether a local business would risk their reputation by despoiling the underpass in this way. I would sincerely hope they wouldn’t.
In whatever light you considered their answer, raising it with them will help ensure it doesn’t happen again, so well done for doing this. It’s hugely frustrating, after the subway was so beautifully cleaned by the volunteers (twice now, if I remember correctly). Thanks for that too!
I too would hold off any naming or shaming, unless it happens again after you’ve raised it with them.