I believe in OP's situation, the owner of the jacket had negative intentions to start with, but I notice around this time of year, overhead bins start to get filled up with (sometimes unusually thick) jackets as well as carry-on bags. As a result, late boarders can't find space. But if the alternative is someone putting their jacket in my seat, please, just put it in the bin. I know suit jackets are a different category -- I wouldn't want them to get wrinkled either -- but it should be taken care of appropriately as well.
An aircraft isn't the place to get into a philosophical argument whether your jacket is a "person" and deserves rights...just don't put it in my seat.