It's not immoral that the parent company was invited to the panel because of their hard work or even possibly paid for their slot (excuse my ignorance, I am not a marketer) and someone comes in and before their message is even delivered they have spam on their chairs? It's sort of the equivalent of a salesman on commission from the car dealership next door to the one you were trying to visit coming over and saying they have just the car for you. It's trespassing, it is definitely immoral. Or, if you aren't convinced it is immoral, it is certainly a dick move.

Wearing a T shirt with a message inviting questions is fine; shoving your message down customer's throats at the expense of your competition is over the line.