It would be possible to encrypt text in such a way that things could be said about its information density but not about its meaning, too, though that would permit steganography to be used. But it would raise the technical bar for using the system for this purpose so high that it would probably drive away all the chickenboners.
There's also a dumb way: length limits. That would force binary data to be divided up into a huge number of posts, making it an annoying medium for file trading. Plotting the Iranian revolution would not require >1mb posts to a forum.