This has been a growing trend where flagship journals are creating separate open-access side-journals.
Usually, the flagship journal is very selective and publishes only research findings applicable to the broadest audience: the catchiest, trendiest, most impactful work, as judged by the editors and reviewers. Since these are very popular journals to read, the publisher can charge high fees to libraries, since every university wants their researchers to have access to the latest and greatest work. Therefore, their costs and profits mostly come from these fees, and they may not need to charge the authors anything.
The secondary journals may be no less scientifically rigorous, yet do not have the restriction of publishing only the most broadly impactful papers, so they don't need to be as selective. That also means they aren't as "must-read", and instead use an open-access model where journal costs and profits come from authors' fees rather than subscribers.
I don't think it's unethical as long as the parameters are made clear, but it is important that researchers familiarize themselves with academic publishing models. There are both benefits (public accessibility, completeness of the academic record) and drawbacks (costs to the author, conflicted incentive structures for rigor) to the paid open access model.