From what you wrote, Scotty, it doesn't sound like they were necessarily being dicks. I would most certainly start by pointing the sign out to them...let them know it's clearly posted. If they don't throw fish back, you don't need to inform them that you're calling TIPS....just call.
Being a dick to them is just situation-dependent, IMHO. Sometimes you need to, sometimes you don't. Calling TIPS is a no-brainer though....always make the call