This discussion of whether the content is legal or not isn't very constructive. Clearly Reddit has to ban illegal content. However, they also have to uphold community standards and enforce rules which keep more toxic parts of the community in line. Allowing pseudo-child porn creates a meeting place for pedophiles and that is likely not a liability that Reddit wants to take on.
Allowing /r/trees creates a meeting place for illegal drug users and that is likely not a liability that Reddit wants to take on. Indeed, quite similar to how people were accused of trading child porn on Reddit, people set up drug deals on Reddit.
The difference is in the effort required to moderate the content. An /r/trees worst case scenario is two people meeting up to perform an illegal activity at a later point. An /r/jailbait worst case scenario is illegal material being indexed by the site.
Additionally, /r/trees isn't generating hundreds of flagged threads for review by the admins (who are then forced to spend man hours browsing pictures of under-aged girls).
