It’s less about the interaction and more about how it’s stored 
In Bear we took the Markdown philosophy all the way: everything lives inside the document; tags, for instance. Folding works the same way: marking a section as folded effectively writes a special comment into the Markdown itself. That’s great in Bear, where the file is ours and lives in our database.
Lettera is different. It opens files that live anywhere on your disk; across projects, and often shared with others or with other tools. Writing Lettera-specific markers into those files, just to remember what’s folded, isn’t something we’re comfortable doing: it would mean putting text only Lettera understands into a file you might hand to someone else (or open in another editor).
We’re taking in a lot of feedback right now, and folding is definitely something we want to discuss soon.