AppleScript bug: When placing plain text on the clipboard, the text is always converted to rich text

Solution 1:

Maybe copying the text to the Pasteboard using Cocoa-AppleScript will only set plain text to paste to the RTF document.

Otherwise, use the following as a workaround:

set testString to "This string should be plain text." as string
do shell script "pbcopy <<<" & quoted form of testString

The previous mentioned workaround method adds a linefeed after testString to the Clipboard and if that's unwanted behavior, use:

set testString to "This string should be plain text." as string
do shell script "printf " & the quoted form of testString & " | pbcopy"

The other option is to use Paste and Match Style ⌥⇧⌘V instead of Paste ⌘V to match what's at the insertion point.