Is it proper to plan have an even at a venue and then ask to guest to pay for their plate?

I agree with you. You need to speak with the ones who make this non-consultative decision and explain that you and other family members would like to be in on any future planning to make sure that a suitable and affordable venue is agreed upon. They may respond by staying that that would be too hard but with free online tools like Doodle it's not that difficult.