What you say is true, but I don't believe a contract of employment entitles any company to tell someone who they can or cannot date.
If, for the reasons you describe, they wish to maintain a public position of "we not encourage dating coworkers" that's one thing, so long as they don't actually think they have any say in this matter.
> I don't believe a contract of employment entitles any company to tell someone who they can or cannot date.
Shame that companies believe that just fine. They can and will fire an employee if "no dating coworkers" is a policy that said employee violated. You are welcome to avoid working for these companies, but that's your only recourse unless you have an overwhelmingly string case that discrimination was involved.
Reasonable is "don't date anyone in your line of management". Options in the case that you do start dating including transferring out to a different manager. That's reasonable, because it's a conflict of interest issue rather than a "who you are allowed to date".
If, for the reasons you describe, they wish to maintain a public position of "we not encourage dating coworkers" that's one thing, so long as they don't actually think they have any say in this matter.