Not disclosing is generally what violates the employment agreements. Most companies realize that relationships happen at work A LOT. Disclosing allows the companies to make sure they avoid any power dynamic issues. I've known many people across many companies who met their future spouses at work.

It looks like there are no laws that restrict US companies from prohibiting romantic relationships between co-workers. (In reverse: US companies are allowed to prohibit romantic relationships between co-workers.) The practicality: If you did restrict it in employee rules, probably people would do it anyway in secret. It is better for HR to know so they can make sure there isn't a supervisory conflict.