How to make webform forget a selected event?
Be very careful when sending people to a webform that has multiple events on them. If they return to that webform (or another webform that pulls in the same Events) at a later time and select another event -> what they are technically doing is canceling their participant registration for the event they un-select and registering themselves for the newly select event.

The setting to not unregister participants from Events will just affect the processing of the form; On load of the webform it does makes sense to me that it shows the Event one is current registered for.