Is it OK for you, a neighbor, and your friend to agree on the toppings of the pizza and then for your neighbor and yourself to "vote in" different toppings for every succeeding pizza?
Your friend consented to the first pizza, but he didn't consent to any pizza after, except insofar as you can say that his remaining a member of the pizza-owners' association represents actual consent.
To oversimplify, yes, the difference is that you've consented to the rules. Imagine you and your friends decide to order a pizza and agree to vote on the toppings. That's fine. But it's not fine for a friend to force you to pay for the pizza when you didn't agree to do so.