Why are locations deleted when an event that uses them is deleted?
well after a bit of experimenting the answer is.. they just are.

Here's a workaround that worked for me.

1 create new location type "location only"
2 make new events of type "location only" and give the title the same as the location you want to manage eg if the location is "Town Hall" then the name of the event will be "Location Town Hall" and the venue will be "Town Hall"

now it doesn't matter what happens with other events that might have used "Town Hall" as their location.. "Town Hall" will survive