AppleScript bug: When placing plain text on the clipboard, the text is always converted to rich text
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 &lt;&lt;&lt;" &amp; 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 " &amp; the quoted form of testString &amp; " | pbcopy"


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