"Hey we think you have great ideas to share, and would really like you to share them."

On short notice: "One of our team members thinks your ideas aren't that important after all, we are going to remove you from the headline. Please keep it short kthxbye."

It shows bad planning, bad organization, lack of cohesion, and outright disrespect of your professional time. You may choose to accept that slap to your face (signaling that it's okay to do that to you), but there's nothing unprofessional about rejecting the attendance and rightfully criticizing the organizers for their rudeness.