I have seen the exact same behavior when importing 4:3 stills with the dimensions of 960×720. FCP apparently automatically flags 1440×1080, 1280×1080 and 960×720 stills as anamorphic, presumably because they match the dimensions of anamorphic video formats like DVCPRO HD and HDV. This automatic flagging happens regardless of sequence settings. If you switch the browser to list view and look at the pixel aspect ratio for your imported stills, it won’t be square pixels. I always work in full-raster HD ProRes sequences and FCP still flags the imported stills as anamorphic.
You can highlight all of your stills in list view in the browser and batch change them to square pixels, but there’s a second quirk: once you’ve corrected the pixel aspect ratio in the browser, if you simply drag from the browser directly to the timeline, the stills will maintain the proper 4:3 aspect ratio and be “pillarboxed” in the 16:9 sequence. However, if you first double-click on a still in the browser (opening it in the viewer), FCP will re-apply the anamorphic stretch to the still.
Unfortunately there is no way that I have found to defeat this behavior. The 1440×1080, 1280×1080 and 960×720 anamorphic flagging seems to be hard coded into the program. It’s pretty annoying, as I frequently edit 720p videos and need to edit in 4:3 960×720 slides, maintaining their proper aspect ratio.
You can do one of two things:
1. You can edit the imported stills into the sequence and then remove the scale and distort attributes for each still in the sequence
2. You can change the pixel aspect ratio of all imported stills to square, then drag them directly into the sequence when needed
I find either method to be a tiresome workaround, and I’m frustrated that there is no way to turn off the automatic anamorphic flagging of stills with dimensions of 1440×1080, 1280×1080 and 960×720. It’s a case of FCP being too smart for its own good.