Even if that's the reason why, it's a pretty brain-dead way for them to implement it. If people want emails from a separate address that CC's Gmail on sending to be seen as sent, Gmail should have them show up like the normal emails that they are received but show a little button that asks if you want them to be seen as sent, and if you accept creates the filter to do so from that source (and shows it in filters so it can be removed later).

It's not like Gmail doesn't show "nudges" and other crap all over the place for stuff like this.