I saw those as regular parser that returns text but not just grep wrapper. Yes It has function named 'grep', but its not grep wrapper. It looks like `lstree` could be combined with `grep` function for emulate `grep -r`.
...for Unix' insistence on composability, the shell tools are often unnecessarily monolithic, probably because that's the only sane way if the only type you have in interconnect is `string`.
Here, you'd need to either parametrize the return type of ls to get simple strings (which is what you want most of time) or additional metadata, or alternatively to have different ls commands.