Have you engaged in these types of “proposals”? Because you’re stating, as a fact, that these kind of arrangements operate without the knowledge of the consenting thirds.
Honesty and transparency, honoring the comfort level of each participant, are two of the primary tenets of open relationships. I want to reassure you that the vast majority of open and polyamorous people are committed to making sure that all people involved are conscious of where they are in space, so to speak.
Not disclosing the state of a primary relationship to a third and being devoid of the intent of letting them be well-informed so that they can choose whether or not they wish to get involved, would generally be considered unethical and non-consensual. Of course, just like there are cheaters in monogamous relationships, there are people who sidestep ethical considerations in all relationship iterations. Liars are everywhere. But the culture of the polyamorous favors an abundance of disclosure.
Maybe this lays your concerns to rest?