Sure they can. But they're probably making money from the arrangement somehow. At least the app publishing fee, but possibly also all data going through their APIs.
https://en.wikipedia.org/wiki/Firebase#User_privacy_controve...
https://www.jurist.org/news/2022/01/us-federal-judge-dismiss...
Period trackers can achieve better prediction by training a model on massive data.
Fitness trackers in general have all sorts of reason to upload data.
The most legitimate reasons are that the dashboards and backups are online.
Fundamentally, the predominant business model is to take people's data as payment.
This continues to be the norm that numbs people into giving it without question or concern.
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How accurate would Apple or Facebook be at predicting someone's period without asking?
This is what I call a Service as a Software Substitute, or SaaSS.
https://www.gnu.org/philosophy/who-does-that-server-really-s...
Volunteers reverse-engineered Bluetooth scales and created all the dashboard you could ever want, and it can export CSV. A company can make the app have a dashboard.
Rather than let unnamed third parties target ads through my weight, I would have returned my scale had this not existed: