I would assume this rule is given by governance for whatever reasons. The limitation applies to every ECG feature for smartwatches independently of the vendor offering it based on my recherche.
Young people quite often a slightly irregular (but normal) heart rate called sinus arrhythmia that the app can’t differentiate from atrial fibrillation. It will take readings but may give you false diagnosis of AF as it can’t assess the baseline of the ECG, only the spikes.