> The nation’s highest court said in the opinion that while “data collection and analysis is a common practice in this digital age,” the sheer size of TikTok and its “susceptibility to foreign adversary control, together with the vast swaths of sensitive data the platform collects” poses a National Security Concern.

FTA

SCOTUS doesn't really get to decide whether that is good justification or even correct

They do, and they did. From the ruling:

The Act’s prohibitions and divestiture requirement are designed to prevent China—a designated foreign adver- sary—from leveraging its control over ByteDance Ltd. to capture the personal data of U. S. TikTok users. This ob- jective qualifies as an important Government interest un- der intermediate scrutiny.