Because it can abused. The easier you make it upload, it also allows bad actors to upload invalid data to cause people to go into quarantine unnecessarily. It only works well if you can trust the data, so I think it should error on the side of validating the data instead of openness.
In an open model, the extent to which abuse is possible would be determined entirely by the authentication requirements (or lack thereof) imposed by the entity operating the server the user selected. That being said, another commenter linked to a set of specifications (https://news.ycombinator.com/item?id=22836871) which seem to indicate that (at least on iOS) the data source is determined entirely by an app that the user chooses to run on top of the framework.
