I'd like to understand the rationale behind this decision. Is there any documentation, talks or discussions about this?
The whole thing was triggered by some discussions where I was mentioning that I enable the blocks behavior for News and Events, because, really, that's what makes sense when you have a Volto website.
I don't think there's any Plone website "in the wild" that enables viewing the same content item with both Volto and Plone classic. We have websites where we use Volto for the whole site and Plone Classic for a "legacy section" that wasn't ported, but viewing the same item in both UIs is a no-no. We've toyed, in the beginning, with making the items compatible and available in both UIs, but the effort wasn't worth it, because, realistically, only website webmaster would be concerned with viewing an item in the old Plone Classic, and even that only in limited cases. Once we grew more comfortable with Volto, we didn't feel the need to view a content item in Plone Classic. As a webmaster, I may use the /edit Plone Classic interface for an item, to double-check some behavior.
In my opinion this would allow a "peacefully" co-existence between Volto and Classic content in the same site. As well as allowing smooth-er migrations from Classic sites to Volto.
I don't think this is a real problem. Yes, it may help co-existence and smoother migrations, but let me tell you, from the experience of migrating several website from Plone classic to Volto, that co-existence is relevant for exactly one or two weeks, while you build a working Volto frontend. After that, nobody will care. Don't complicate it, there's no imaginary developer or website that would be helped by making these two worlds, the Classic UI and Volto, more compatible with each other. Once you make a decision to move in Volto's direction, there's no going back. You may decide, in the beginning, to stick with Classic, which is fine, hopefully, at one point, the Volto frontend would provide enough of a value proposition that it would be a good idea to migrate.