I wasn't sure if I should call this a bug or not so I'll make this a feature request :-)
The "Force Overwrite Thumb" option is obviously mean for scraping movies. If I go out of my way to use the "URL or Path" button and change the image I think I have pretty much confirmed my intention and that preference should be ignored.
BTW: I'm not big on movie terminology so there may be a difference but from what I can see you are using "thumb" and "poster" to mean the same thing? If so can we use one word as I get easily confused!