Just played around with it myself (don't usually use the GUI very often) and it's pretty misleading.

By default, when selecting a backup file that has the same DB name as one that exists on that server, if you don't go onto the options tab, you'll put your source database in the RESTORING state as it does a tail log backup of it, even if your destination name is different.

It's not actually restoring over the top (you can still bring it back ONLINE), but can see why you'd think it was restoring over the top!