Image colors from imported TIFF or PICT images cannot be cut or deleted from their palette, even when you delete the imported image from your document.
The following steps will recreate the behavior:
1) Import a color TIFF or PICT image
2) With the image selected, choose Image Colors from the Special menu in the Color Palette
3) Delete the image
4) Attempt to delete or cut colors in the color palette
You won't be able to do this. You can, however, copy the colors to another color palette, then delete the palette with the image colors.