That's because of the widget you are using. It's used for single attributes. How should Mendix or the client know which of all the associated objects you need?
So either give each Tag-object a unique identifier to be able to create seperate dataviews for each object. This approach however limits you to a specific amount of dataviews, it's not flexible.
So the best way to work with a *-* or 1-* association is to use a grid, listview or template grid to show multiple objects (Tags) associated to the one object shown (Event). In this case I'd say a template grid should be most flexible to show it in a way that suits your needs.