The "rich" property was originally added to allow an arbitrary limit on the amount of text (see
versions.txt) when used in combination with the "limit" property. It was not actually related to the use of RTF text.
When I added the "rich" property, I enabled an editbox setting that allowed only plain text to be used and not RTF text. Unfortunately the setting was not applied correctly. This allowed scripters to use RTF text.
To add to the confusion, the Unicode version of the richedit editbox only interprets RTF text if it is in ANSI single-byte UTF-8 format.
So, in order to maintain backwards compatibility with an unintentional feature caused by a subtle bug in code that was added to support an altogether different feature, I will have to make some rather tricky changes :-)
This is on my to-do list but it is not a priority at this time.