Legally, probably. But in the US, "legally" is enforced by the executive, and that's who's telling them to comply.

To be clear, Security Clearances is just an EO thing.

The actual laws just speak about things being critical to national security and don't use the words like "Top Secret" and etc.

The whole actually labeling things "Top Secret" and etc started off with Clinton [1] but then Bush [2] and Obama [3] modified the rules around classification.

[1]: https://en.wikipedia.org/wiki/Executive_Order_12958

[2]: https://en.wikipedia.org/wiki/Executive_Order_13292

[3]: https://en.wikipedia.org/wiki/Executive_Order_13526