In the treatment Task list, by default, all animals are shown on this list, including animals in foster. To only show animals in the shelter, change location to "Onsite only" or to the specific location you want to see.
If you want to mark all treatments in foster as being completed, choose location as "In Foster only" and scroll to the bottom of the page to load all treatments. Click on the "select all" checkbox in the top-left corner of the task list table, and then click on "Complete selected". You can enter "Foster parent" as the "Given by" value.