Why are non-members being asked to fill out a membership form when they buy an event ticket?

There is an option when setting up an event to make it an Open Event.  If you tick that option, then any person buying a ticket for the event will not be asked to complete a membership form. 

If you leave the Open Event box unticked, then any person who has not previously completed a membership form will be presented with your membership form.

 

Open Event - click to enlarge